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/tests/common/mod.rs

@@ -74,6 +74,10 @@ fn harness_with_auth_config(
             max_file_bytes: 256 * 1024,
         },
         auth_limiter,
+        // Default harness has no crawler daemon wired up; admin resync
+        // handlers return 503 in this config. Tests that need a stub
+        // resync service swap it in via `harness_with_resync`.
+        resync: None,
     };
     Harness { app: router(state), _storage_dir: storage_dir }
 }
@@ -124,6 +128,37 @@ pub fn harness_with_auth_rate_limit(
     harness_with_auth_config(pool, storage, storage_dir, auth)
 }
 
+/// Like [`harness`] but slots a caller-supplied [`ResyncService`] stub
+/// into `AppState.resync`. Used by the admin resync tests so the
+/// endpoint path is exercised without standing up a real Chromium.
+pub fn harness_with_resync(
+    pool: PgPool,
+    resync: Arc<dyn mangalord::crawler::resync::ResyncService>,
+) -> Harness {
+    let storage_dir = tempfile::tempdir().expect("tempdir");
+    let storage = Arc::new(LocalStorage::new(storage_dir.path()));
+    let auth = AuthConfig {
+        cookie_secure: false,
+        ..AuthConfig::default()
+    };
+    let auth_limiter = Arc::new(AuthRateLimiter::new(auth.rate_limit));
+    let state = AppState {
+        db: pool,
+        storage,
+        auth,
+        upload: UploadConfig {
+            max_request_bytes: 4 * 1024 * 1024,
+            max_file_bytes: 256 * 1024,
+        },
+        auth_limiter,
+        resync: Some(resync),
+    };
+    Harness {
+        app: router(state),
+        _storage_dir: storage_dir,
+    }
+}
+
 /// Wraps a real `Storage` and fails on the N-th `put` call so tests can
 /// assert that handlers roll their DB writes back when storage errors
 /// mid-upload. Reads and other operations delegate to `inner`.