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/repo/crawler.rs

@@ -458,6 +458,44 @@ pub async fn sync_manga_chapters(
     Ok(diff)
 }
 
+/// Count the chapters that the source `(source_id, source_manga_key)`
+/// is currently known to attach to — i.e. the number of `chapter_sources`
+/// rows for the manga identified by the (source_id, source_manga_key)
+/// pair, restricted to live (`dropped_at IS NULL`) rows.
+///
+/// Used by the metadata pass's partial-render guard: if `fetch_manga`
+/// returns an empty `chapters` Vec but the source previously surfaced
+/// chapters here, that's most likely a chromium snapshot taken between
+/// the `#chapter_table` wrapper render and its rows render — the
+/// safest move is to skip `sync_manga_chapters` so the soft-drop
+/// branch doesn't flip every existing chapter to `dropped_at`.
+///
+/// Returns `Ok(0)` when the manga is brand-new (no `manga_sources`
+/// row yet), which is the legitimate "this manga has no chapters yet"
+/// case and must NOT be flagged.
+pub async fn live_chapter_count_for_source_manga(
+    pool: &PgPool,
+    source_id: &str,
+    source_manga_key: &str,
+) -> sqlx::Result<i64> {
+    let row: Option<(i64,)> = sqlx::query_as(
+        "SELECT COUNT(*) \
+           FROM chapter_sources cs \
+           JOIN chapters c ON c.id = cs.chapter_id \
+           JOIN manga_sources ms \
+             ON ms.manga_id = c.manga_id \
+            AND ms.source_id = cs.source_id \
+          WHERE ms.source_id = $1 \
+            AND ms.source_manga_key = $2 \
+            AND cs.dropped_at IS NULL",
+    )
+    .bind(source_id)
+    .bind(source_manga_key)
+    .fetch_optional(pool)
+    .await?;
+    Ok(row.map(|(n,)| n).unwrap_or(0))
+}
+
 /// Mark a metadata pass as in-flight for `source_id`. Stamps
 /// `last_run_completed:<source_id>` in `crawler_state` with
 /// `{"completed": false, "at": now}`. A crash, panic, or SIGKILL after