fix(crawler): close walker race against site reordering (0.35.1)

The target site orders by update_date DESC, and any new or updated
manga pushes everyone down by one slot. The paginated walker was
blind to this drift:

* Backfill (page last -> 1): shifts push items into pages already
  finished. The displaced manga was silently missed; with
  mark_dropped_mangas running on a fully-completed walk, items even
  got false-dropped because last_seen_at was stale.
* Incremental (page 1 -> last): a shift causes the slot-last item
  of an already-read page to reappear on the next page, leading to
  a redundant fetch_manga and an inflated consecutive_unchanged
  streak.

Fix is two-pronged:

1. Backfill boundary re-check. After fetching each page P, re-fetch
   the previously-walked page P+1 and check where its old slot-0
   key now sits. If it slid to slot K, the first K entries are
   items that used to live on P and slid past us; they get appended
   to the batch. If the anchor is gone entirely (multi-page shift
   or it was bumped to page 1), the whole re-fetched page is
   processed conservatively and the pipeline dedup absorbs the
   noise. The re-check must be the *last* navigation of the
   iteration to close the within-iteration race.

2. Run-scoped dedup in run_metadata_pass. A HashSet<String> of
   source_manga_keys avoids double-processing. The set uses a
   contains-then-insert pattern with insert firing *after* a
   successful upsert, so a transient fetch/upsert failure leaves
   the key retryable if it reappears later in the same pass (via
   the boundary re-check or another batch).

Incremental mode does not run the re-check (shifts move in the
same direction as the walk); only the dedup helps it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/crawler/source/target.rs

@@ -103,6 +103,7 @@ impl Source for TargetSource {
             backfill,
             pages_remaining: order,
             first_page_html: Some(first_html),
+            prev: None,
         }))
     }
 
@@ -160,6 +161,13 @@ struct TargetSourceWalker {
     backfill: bool,
     pages_remaining: VecDeque<i32>,
     first_page_html: Option<String>,
+    /// Page number and slot-0 `source_manga_key` of the previously-walked
+    /// page. Updated after every batch (cheap, unconditional) but only
+    /// *read* by the boundary re-check, which itself runs only in backfill
+    /// mode. A single `Option` so the half-set state (`page_num` known but
+    /// `key` not, or vice versa) is unrepresentable; `None` here suppresses
+    /// the next iteration's re-check (no anchor to compare against).
+    prev: Option<(i32, String)>,
 }
 
 #[async_trait]
@@ -206,10 +214,77 @@ impl DiscoverWalk for TargetSourceWalker {
             )
             .await?
         };
+        // Capture slot-0 of the page as the site presents it (newest first)
+        // *before* the backfill `.reverse()` below — after reversal slot 0
+        // is the oldest entry, which would defeat the next iteration's
+        // boundary re-check.
+        let current_first = page_refs.first().map(|r| r.source_manga_key.clone());
+
+        // Boundary re-check (backfill only). The site orders by update_date
+        // DESC, so shifts during the walk push items from low-numbered pages
+        // to high-numbered ones — into pages backfill has already finished.
+        // After fetching this page, re-fetch the *previous* iteration's page
+        // and look for items that slid past us mid-walk. Must be the last
+        // navigation of the iteration to close the within-iteration race.
+        let mut displaced: Vec<SourceMangaRef> = Vec::new();
+        if self.backfill {
+            if let Some((prev_page_num, prev_first_key)) = self.prev.clone() {
+                match recheck_prev_page(ctx, &self.base_url, prev_page_num).await {
+                    Ok(refetched) => {
+                        let (d, outcome) = detect_displaced(&prev_first_key, &refetched);
+                        match outcome {
+                            DisplacementOutcome::NoShift => {}
+                            DisplacementOutcome::Shifted(k) => {
+                                tracing::info!(
+                                    page_num,
+                                    prev_page_num,
+                                    k,
+                                    "boundary re-check: shift detected, recovering displaced refs"
+                                );
+                            }
+                            DisplacementOutcome::NotFoundFallback => {
+                                tracing::warn!(
+                                    page_num,
+                                    prev_page_num,
+                                    prev_first_key = %prev_first_key,
+                                    refetched_len = refetched.len(),
+                                    "boundary re-check: prev_first not found, falling back to full re-process"
+                                );
+                            }
+                        }
+                        displaced = d;
+                    }
+                    Err(e) => {
+                        tracing::warn!(
+                            page_num,
+                            prev_page_num,
+                            error = ?e,
+                            "boundary re-check: re-fetch failed, skipping check for this boundary"
+                        );
+                    }
+                }
+            }
+        }
+
+        // Remember the boundary for the next iteration. An empty `page_refs`
+        // yields `None`, which intentionally suppresses the next re-check
+        // (no anchor to compare against).
+        self.prev = current_first.map(|key| (page_num, key));
+
         if self.backfill {
             page_refs.reverse();
         }
-        tracing::info!(page_num, count = page_refs.len(), "page walked");
+        // Append displaced refs to the end of the batch. Order doesn't
+        // affect backfill semantics (no `consecutive_unchanged` streak in
+        // this mode), and the pipeline-level dedup set handles any overlap.
+        let displaced_count = displaced.len();
+        page_refs.extend(displaced);
+        tracing::info!(
+            page_num,
+            count = page_refs.len(),
+            displaced = displaced_count,
+            "page walked"
+        );
         Ok(Some(page_refs))
     }
 }
@@ -592,6 +667,67 @@ fn compute_metadata_hash(m: &SourceManga) -> String {
     format!("{:x}", h.finalize())
 }
 
+/// Outcome of a boundary re-check, surfaced for telemetry + tests. The
+/// caller's recovery action is determined by the returned `Vec`; this enum
+/// only labels which branch fired so logging and assertions can distinguish
+/// "site is stable" from "we papered over a shift" from "we fell back".
+#[derive(Debug, PartialEq, Eq)]
+enum DisplacementOutcome {
+    /// `prev_first` is still at slot 0 of the re-fetched page — no shift
+    /// happened between the prior iteration and this one's re-check.
+    NoShift,
+    /// `prev_first` slid down to slot `K`; the first `K` entries are items
+    /// that used to live on the page we just walked.
+    Shifted(usize),
+    /// `prev_first` is gone from the re-fetched page — multiple pages-worth
+    /// of shifts happened, or it was bumped to page 1. Treat all refetched
+    /// entries as potentially displaced; the pipeline-level dedup absorbs
+    /// the noise.
+    NotFoundFallback,
+}
+
+/// Compare a previously-walked page's slot-0 key against a fresh fetch of
+/// that page. Returns the entries that appear *ahead* of `prev_first` in
+/// the re-fetched page — items that slid in from the page the caller is
+/// currently processing.
+fn detect_displaced(
+    prev_first: &str,
+    refetched: &[SourceMangaRef],
+) -> (Vec<SourceMangaRef>, DisplacementOutcome) {
+    let Some(k) = refetched
+        .iter()
+        .position(|r| r.source_manga_key == prev_first)
+    else {
+        return (refetched.to_vec(), DisplacementOutcome::NotFoundFallback);
+    };
+    if k == 0 {
+        (Vec::new(), DisplacementOutcome::NoShift)
+    } else {
+        (refetched[..k].to_vec(), DisplacementOutcome::Shifted(k))
+    }
+}
+
+/// Re-fetch a previously-walked listing page to feed [`detect_displaced`].
+/// Uses the same retry chain as the primary page fetch in `next_batch` so
+/// a transient hiccup doesn't tank an entire backfill walk.
+async fn recheck_prev_page(
+    ctx: &FetchContext<'_>,
+    base_url: &str,
+    page_num: i32,
+) -> Result<Vec<SourceMangaRef>, PageError> {
+    retry_on_transient(
+        || async {
+            let url = page_url(base_url, page_num);
+            let html = navigate(ctx, &url).await?;
+            let doc = scraper::Html::parse_document(&html);
+            parse_manga_list_from(&doc)
+        },
+        PAGE_TRANSIENT_RETRY_ATTEMPTS,
+        PAGE_TRANSIENT_RETRY_DELAY,
+    )
+    .await
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -990,4 +1126,100 @@ mod tests {
         let incremental = build_page_order(Some(1), false);
         assert_eq!(Vec::from(incremental), vec![1]);
     }
+
+    fn make_ref(key: &str) -> SourceMangaRef {
+        SourceMangaRef {
+            source_manga_key: key.to_string(),
+            title: key.to_string(),
+            url: format!("https://target.example/manga/{key}"),
+        }
+    }
+
+    #[test]
+    fn detect_displaced_no_shift_when_prev_first_still_at_slot_zero() {
+        let refetched = vec![make_ref("A"), make_ref("B"), make_ref("C")];
+        let (displaced, outcome) = detect_displaced("A", &refetched);
+        assert!(displaced.is_empty());
+        assert_eq!(outcome, DisplacementOutcome::NoShift);
+    }
+
+    #[test]
+    fn detect_displaced_one_shift_returns_single_intruder() {
+        let refetched = vec![make_ref("X"), make_ref("A"), make_ref("B")];
+        let (displaced, outcome) = detect_displaced("A", &refetched);
+        assert_eq!(displaced.len(), 1);
+        assert_eq!(displaced[0].source_manga_key, "X");
+        assert_eq!(outcome, DisplacementOutcome::Shifted(1));
+    }
+
+    #[test]
+    fn detect_displaced_multi_shift_returns_all_intruders() {
+        let refetched = vec![
+            make_ref("X1"),
+            make_ref("X2"),
+            make_ref("X3"),
+            make_ref("A"),
+            make_ref("B"),
+            make_ref("C"),
+        ];
+        let (displaced, outcome) = detect_displaced("A", &refetched);
+        let keys: Vec<&str> = displaced
+            .iter()
+            .map(|r| r.source_manga_key.as_str())
+            .collect();
+        assert_eq!(keys, vec!["X1", "X2", "X3"]);
+        assert_eq!(outcome, DisplacementOutcome::Shifted(3));
+    }
+
+    #[test]
+    fn detect_displaced_full_page_shift_returns_all_but_last() {
+        // `prev_first` at the last slot — every preceding entry is an
+        // intruder shifted in from the page the caller is processing.
+        let mut refetched: Vec<_> = (0..9).map(|i| make_ref(&format!("X{i}"))).collect();
+        refetched.push(make_ref("A"));
+        let (displaced, outcome) = detect_displaced("A", &refetched);
+        assert_eq!(displaced.len(), 9);
+        assert_eq!(outcome, DisplacementOutcome::Shifted(9));
+    }
+
+    #[test]
+    fn detect_displaced_not_found_returns_full_page_for_conservative_recovery() {
+        // > page-worth of shifts (or `prev_first` itself was bumped to
+        // page 1): can't pinpoint K, fall back to "process everything";
+        // pipeline dedup absorbs the noise.
+        let refetched = vec![make_ref("Y"), make_ref("Z")];
+        let (displaced, outcome) = detect_displaced("A", &refetched);
+        let keys: Vec<&str> = displaced
+            .iter()
+            .map(|r| r.source_manga_key.as_str())
+            .collect();
+        assert_eq!(keys, vec!["Y", "Z"]);
+        assert_eq!(outcome, DisplacementOutcome::NotFoundFallback);
+    }
+
+    #[test]
+    fn detect_displaced_empty_page_returns_empty_with_fallback_outcome() {
+        // Re-fetch came back empty (transient mimicry or last-page tail).
+        // No anchor means we can't classify; fall back is the safe label.
+        let (displaced, outcome) = detect_displaced("A", &[]);
+        assert!(displaced.is_empty());
+        assert_eq!(outcome, DisplacementOutcome::NotFoundFallback);
+    }
+
+    #[test]
+    fn detect_displaced_takes_first_occurrence_when_key_repeats() {
+        // Defensive: if the source ever returns the same key twice on a
+        // page, anchoring on the first match keeps the displaced slice
+        // bounded and deterministic.
+        let refetched = vec![
+            make_ref("X"),
+            make_ref("A"),
+            make_ref("Y"),
+            make_ref("A"),
+        ];
+        let (displaced, outcome) = detect_displaced("A", &refetched);
+        assert_eq!(displaced.len(), 1);
+        assert_eq!(displaced[0].source_manga_key, "X");
+        assert_eq!(outcome, DisplacementOutcome::Shifted(1));
+    }
 }