feat: cover retry backfill + admin force-resync for manga & chapter (0.50.0)

Adds a per-tick cover-backfill pass to the crawler daemon so mangas whose
cover download failed on first attempt get retried — the metadata pass's
early-stop optimisation otherwise prevents the walk from revisiting them.

Adds admin-only POST /admin/mangas/:id/resync and POST /admin/chapters/:id/resync
that refetch metadata + cover (or chapter content with force_refetch) from the
crawler source synchronously and return the refreshed row. Surfaced in the
UI as "Force resync" buttons on the manga detail and reader pages,
admin-only via session.user.is_admin.

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

backend/src/api/admin/mod.rs

@@ -5,6 +5,7 @@
 //! `crate::auth::extractor::RequireAdmin`).
 
 pub mod mangas;
+pub mod resync;
 pub mod system;
 pub mod users;
 
@@ -16,5 +17,6 @@ pub fn routes() -> Router<AppState> {
     Router::new()
         .merge(users::routes())
         .merge(mangas::routes())
+        .merge(resync::routes())
         .merge(system::routes())
 }