feat(api): admin manga/chapter overview with derived sync state (0.39.0)

Adds GET /api/v1/admin/mangas and /admin/mangas/:id/chapters guarded by
RequireAdmin. Sync state is computed at query time from the existing
crawler signals (manga_sources / chapter_sources / crawler_jobs) — no
new state column is persisted, so the crawler stays the single writer
of these signals.

Per-manga priority: InProgress (in-flight sync_manga or
sync_chapter_list job) > Dropped (all source rows soft-dropped) >
Synced (default; covers user-uploaded mangas with zero source rows).

Per-chapter priority: Downloading (in-flight sync_chapter_content) >
Dropped (all source rows soft-dropped) > Failed (most-recent terminal
job is dead) > NotDownloaded (page_count = 0) > Synced. The Failed
check sits ABOVE NotDownloaded so the more informative "we tried and
it died" state wins over "we never got around to it" — see the
priority comment in repo/admin_view.rs.

Migration 0020 adds a partial index on
crawler_jobs((payload->>'source_manga_key')) for the one job kind
(sync_manga) whose payload doesn't carry manga_id directly — without it
the in-flight detection for a manga falls back to a seqscan over the
job table.

backend/migrations/0020_admin_jobs_payload_index.sql

@@ -0,0 +1,14 @@
+-- Per-manga sync-state derivation joins crawler_jobs to manga_sources via
+-- (payload->>'source_id', payload->>'source_manga_key') for the
+-- `sync_manga` job kind (whose payload doesn't carry a manga_id directly).
+-- Without this index the join falls back to a seqscan of crawler_jobs on
+-- every admin manga listing — a noticeable cost as the job table grows
+-- with the daily metadata pass.
+--
+-- Partial on `state IN ('pending','running')` so it covers only in-flight
+-- jobs (the bulk of the table is done/dead and irrelevant to "is this
+-- manga being synced right now").
+CREATE INDEX crawler_jobs_sync_manga_key_idx
+    ON crawler_jobs ((payload->>'source_manga_key'))
+ WHERE state IN ('pending', 'running')
+   AND payload->>'kind' = 'sync_manga';