fix(crawler): skip sync when empty chapters + prior > 0 (0.36.6)

The wait_for_selector wait in 0.36.2 narrows the partial-render race
window but doesn't close it: a render that takes longer than
SELECTOR_TIMEOUT (10s) still hands an empty Vec to sync_manga_chapters,
and the soft-drop branch flips every existing chapter to dropped_at.
The next tick recovers but a manga's reader briefly stops working in
between.

Close it at the pipeline level. Between fetch_manga and the upsert/
sync, if the parsed chapter list is empty and the prior live count
for (source_id, source_manga_key) is > 0, treat the fetch as a
transient failure: log, bump mangas_failed, skip upsert + sync + the
seen.insert so a later batch / tick retries. Brand-new mangas with
genuinely zero chapters (prior == 0) pass through unchanged.

New repo helper repo::crawler::live_chapter_count_for_source_manga
joins chapters → chapter_sources → manga_sources with dropped_at IS
NULL — same lockstep as dispatch_target and the enqueue queries.

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

backend/src/crawler/pipeline.rs

@@ -215,6 +215,48 @@ pub async fn run_metadata_pass(
                 }
             };
 
+            // Partial-render guard: an empty chapter list paired with a
+            // prior count > 0 is overwhelmingly a chromium snapshot
+            // taken between the #chapter_table wrapper render and its
+            // rows render. The wait_for_selector wait in `navigate`
+            // narrows this window but cannot close it for slow renders
+            // beyond the selector budget. Treat as a transient failure
+            // here — skip upsert, skip seen.insert — so the next batch
+            // (or the next tick) retries. Skipped in `skip_chapters`
+            // mode because the parser is configured to return an empty
+            // Vec by design there.
+            if !skip_chapters && manga.chapters.is_empty() {
+                match repo::crawler::live_chapter_count_for_source_manga(
+                    db, source_id, &r.source_manga_key,
+                )
+                .await
+                {
+                    Ok(prior) if prior > 0 => {
+                        tracing::warn!(
+                            key = %r.source_manga_key,
+                            url = %r.url,
+                            prior_chapter_count = prior,
+                            "fetch_manga returned empty chapters but prior count > 0; treating as partial-render transient and skipping"
+                        );
+                        stats.mangas_failed += 1;
+                        continue;
+                    }
+                    Ok(_) => {}
+                    Err(e) => {
+                        // DB lookup failed — fail safe: skip rather
+                        // than risk a soft-drop on a manga whose prior
+                        // count we couldn't confirm.
+                        tracing::warn!(
+                            key = %r.source_manga_key,
+                            error = ?e,
+                            "live_chapter_count_for_source_manga failed; skipping cautiously"
+                        );
+                        stats.mangas_failed += 1;
+                        continue;
+                    }
+                }
+            }
+
             let upsert = match repo::crawler::upsert_manga_from_source(
                 db, source_id, &r.url, &manga,
             )