fix(crawler): serialize sync_manga_chapters per-manga (0.35.6)

Two concurrent calls of sync_manga_chapters for the same manga both
read seen_keys, both run the drop UPDATE filtered on `NOT (key = ANY
$3)`, and the later commit can soft-drop a chapter the earlier had
just inserted (lost-update under MVCC). Today the cron tick is the
only caller and the daemon-level advisory lock keeps it single-flight,
but that lock is held on one pool connection and doesn't actually
serialize the *function*: any future caller (bookmark hook,
admin-triggered re-sync, parallel worker) would race against the cron.

Add `pg_advisory_xact_lock(hashtextextended(manga_id::text, 0))` at
the start of the transaction. Auto-releases on commit/rollback so a
panic mid-call can't strand the lock. Lock keyed per-manga so calls
for different mangas still parallelize.

Test sync_chapters_serializes_concurrent_calls_for_same_manga spawns
two tokio tasks calling the function concurrently with overlapping
chapter lists and asserts every chapter survives.

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

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.35.5"
+version = "0.35.6"
 edition = "2021"
 default-run = "mangalord"