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/crawler_sync.rs

@@ -829,6 +829,107 @@ async fn sync_tags_garbage_collects_orphan_user_attachments(pool: PgPool) {
     assert_eq!(orphan_rows, 0, "orphan user-attached tag should be reaped");
 }
 
+// ---- list_missing_covers ---------------------------------------------------
+
+#[sqlx::test(migrations = "./migrations")]
+async fn list_missing_covers_only_returns_rows_without_cover(pool: PgPool) {
+    crawler::ensure_source(&pool, "target", "T", "https://x.example")
+        .await
+        .unwrap();
+    let with_cover = sample_manga("with", "With Cover", "h1");
+    let without_cover = sample_manga("without", "No Cover", "h2");
+    let _w = crawler::upsert_manga_from_source(&pool, "target", "https://x.example/with", &with_cover)
+        .await
+        .unwrap();
+    let nc = crawler::upsert_manga_from_source(&pool, "target", "https://x.example/without", &without_cover)
+        .await
+        .unwrap();
+
+    // Manually set a cover for `with` only.
+    sqlx::query("UPDATE mangas SET cover_image_path = 'mangas/x/cover.jpg' WHERE id = $1")
+        .bind(_w.manga_id)
+        .execute(&pool)
+        .await
+        .unwrap();
+
+    let entries = crawler::list_missing_covers(&pool, 50).await.unwrap();
+    assert_eq!(entries.len(), 1, "exactly the manga without a cover");
+    assert_eq!(entries[0].manga_id, nc.manga_id);
+    assert_eq!(entries[0].source_manga_key, "without");
+    assert_eq!(entries[0].source_url, "https://x.example/without");
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn list_missing_covers_skips_dropped_source_rows(pool: PgPool) {
+    crawler::ensure_source(&pool, "target", "T", "https://x.example")
+        .await
+        .unwrap();
+    let m = sample_manga("foo", "Foo", "h1");
+    let up = crawler::upsert_manga_from_source(&pool, "target", "https://x.example/foo", &m)
+        .await
+        .unwrap();
+    sqlx::query("UPDATE manga_sources SET dropped_at = NOW() WHERE manga_id = $1")
+        .bind(up.manga_id)
+        .execute(&pool)
+        .await
+        .unwrap();
+
+    let entries = crawler::list_missing_covers(&pool, 50).await.unwrap();
+    assert!(
+        entries.is_empty(),
+        "dropped-source mangas must not be backfilled — no live source to fetch from"
+    );
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn list_missing_covers_respects_limit(pool: PgPool) {
+    crawler::ensure_source(&pool, "target", "T", "https://x.example")
+        .await
+        .unwrap();
+    for i in 0..5 {
+        let key = format!("m{i}");
+        let url = format!("https://x.example/{key}");
+        let m = sample_manga(&key, &format!("M{i}"), &format!("h{i}"));
+        let _ = crawler::upsert_manga_from_source(&pool, "target", &url, &m)
+            .await
+            .unwrap();
+    }
+    let entries = crawler::list_missing_covers(&pool, 3).await.unwrap();
+    assert_eq!(entries.len(), 3, "limit caps the result set");
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn list_missing_covers_deduplicates_per_manga(pool: PgPool) {
+    // A manga surfaced by two sources should produce ONE backfill
+    // entry, not two — otherwise the per-tick cap could be eaten by
+    // duplicates and starve other mangas.
+    crawler::ensure_source(&pool, "src-a", "A", "https://a.example")
+        .await
+        .unwrap();
+    crawler::ensure_source(&pool, "src-b", "B", "https://b.example")
+        .await
+        .unwrap();
+    let m = sample_manga("foo", "Foo", "h1");
+    let up = crawler::upsert_manga_from_source(&pool, "src-a", "https://a.example/foo", &m)
+        .await
+        .unwrap();
+    // Second source attaches to the SAME manga row.
+    sqlx::query(
+        "INSERT INTO manga_sources (source_id, source_manga_key, manga_id, source_url) \
+         VALUES ($1, $2, $3, $4)",
+    )
+    .bind("src-b")
+    .bind("foo-on-b")
+    .bind(up.manga_id)
+    .bind("https://b.example/foo")
+    .execute(&pool)
+    .await
+    .unwrap();
+
+    let entries = crawler::list_missing_covers(&pool, 50).await.unwrap();
+    assert_eq!(entries.len(), 1, "DISTINCT ON (m.id) collapses duplicate source rows");
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn re_appearing_manga_clears_dropped_at(pool: PgPool) {
     crawler::ensure_source(&pool, "target", "T", "https://x.example")