fix(admin): three review findings — audit no-op, 404, chapter priority (0.41.1)

- admin_safe_set_is_admin: short-circuit when target.is_admin == value,
  before writing audit. PATCH {is_admin: true} on someone already admin
  previously wrote a misleading "promote_user" row even though the UPDATE
  was a no-op.

- list_chapters (/admin/mangas/:id/chapters): explicit exists() check on
  manga_id, returns 404 instead of 200 [] for a typo'd / deleted manga.

- ChapterSyncState priority: the Failed branch now requires page_count = 0,
  so a chapter with pages on disk AND a historical dead job (from a
  re-download attempt that crashed) stays Synced. The old order
  contradicted Synced's documented "downloaded at some point" contract.
  Doc comments updated alongside the SQL.

Three new regression tests pin the behaviour.

backend/src/domain/sync_state.rs

@@ -31,10 +31,12 @@ pub enum ChapterSyncState {
     /// At least one `chapter_sources` row exists AND all of them are
     /// `dropped_at IS NOT NULL`.
     Dropped,
-    /// Most recent `sync_chapter_content` job for this chapter is `dead`
-    /// (terminal failure). Checked BEFORE `NotDownloaded` so the more
+    /// `page_count = 0` AND a `dead` `sync_chapter_content` job exists
+    /// for this chapter. Checked BEFORE `NotDownloaded` so the more
     /// informative "we tried and it died" state wins over "we never
-    /// got around to it".
+    /// got around to it". Does NOT fire when `page_count > 0`, because
+    /// pages on disk mean the chapter IS synced regardless of historical
+    /// job failures — see the priority comment in `repo::admin_view`.
     Failed,
     /// `page_count = 0` and no in-flight or failed job — the chapter
     /// row exists but content has never been downloaded.