fix(correctness): pagination tiebreakers, dedup race, duration gating (0.87.6)

Three medium correctness fixes:

1. **Pagination ORDER BY missing tiebreakers** — four admin list queries
   sorted by a timestamp or `chapters.number` with no stable secondary
   sort. Add `id` tiebreakers across `repo::admin_view`,
   `repo::admin_audit`, `repo::bookmark`.

2. **`enqueue_pages` NOT EXISTS read-then-insert race** — concurrent
   admin clicks could land duplicate analyze_page jobs. Migration 0031
   adds a partial unique index mirroring 0014; query relies on
   `ON CONFLICT DO NOTHING`.

3. **`record_duration` overwrote a prior done row's duration on a
   non-terminal failed retry of a force-re-analyze.** Gate the call
   on "did this attempt actually write a row?".

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

backend/src/repo/admin_view.rs

@@ -239,7 +239,11 @@ pub async fn list_mangas_with_sync_state(
         )
         SELECT * FROM classified
          WHERE ($2::text IS NULL OR sync_state = $2)
-         ORDER BY updated_at DESC
+         -- `id` tiebreaker stabilises pagination across mangas that share an
+         -- `updated_at` (multi-row crawl-tick bulk updates land within the
+         -- same statement_timestamp() and tie on Postgres' default ts
+         -- resolution). Without it, page boundaries can skip or repeat rows.
+         ORDER BY updated_at DESC, id DESC
          LIMIT $3 OFFSET $4
         "#,
         case = MANGA_SYNC_STATE_CASE
@@ -332,7 +336,12 @@ pub async fn list_chapters_with_sync_state(
               WHERE cs.chapter_id = c.id AND cs.dropped_at IS NULL) AS latest_seen_at
           FROM chapters c
          WHERE c.manga_id = $1
-         ORDER BY c.number ASC
+         -- `id` tiebreaker: migration 0013 dropped the (manga_id, number)
+         -- UNIQUE constraint to allow variants ("Ch.14: PH" / "Ch.14:
+         -- Official") and non-numeric entries (all parse to 0). Without a
+         -- tiebreaker, pagination over those tied rows can skip or repeat
+         -- depending on Postgres' chosen sort order.
+         ORDER BY c.number ASC, c.id ASC
          LIMIT $2 OFFSET $3
         "#,
     )