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/repo/crawler.rs

@@ -542,6 +542,51 @@ pub async fn mark_run_completed(pool: &PgPool, source_id: &str) -> sqlx::Result<
     Ok(())
 }
 
+/// List mangas whose `cover_image_path IS NULL` but a live
+/// `manga_sources` row still attaches them to a source. The bounded
+/// result feeds the cover-backfill pass in [`crate::crawler::pipeline`]:
+/// each entry is one (manga, freshest source row) pair where a cover
+/// re-download is in order.
+///
+/// Per-manga deduplication uses `DISTINCT ON (m.id)` keyed on the row
+/// with the newest `last_seen_at`, so a manga that's surfaced by
+/// multiple sources only produces one row (the freshest). Sort is
+/// stable for tests.
+pub async fn list_missing_covers(
+    pool: &PgPool,
+    max: i64,
+) -> sqlx::Result<Vec<MissingCoverEntry>> {
+    let rows: Vec<(Uuid, String, String)> = sqlx::query_as(
+        r#"
+        SELECT DISTINCT ON (m.id) m.id, ms.source_manga_key, ms.source_url
+          FROM mangas m
+          JOIN manga_sources ms ON ms.manga_id = m.id
+         WHERE m.cover_image_path IS NULL
+           AND ms.dropped_at IS NULL
+         ORDER BY m.id, ms.last_seen_at DESC
+         LIMIT $1
+        "#,
+    )
+    .bind(max)
+    .fetch_all(pool)
+    .await?;
+    Ok(rows
+        .into_iter()
+        .map(|(manga_id, source_manga_key, source_url)| MissingCoverEntry {
+            manga_id,
+            source_manga_key,
+            source_url,
+        })
+        .collect())
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct MissingCoverEntry {
+    pub manga_id: Uuid,
+    pub source_manga_key: String,
+    pub source_url: String,
+}
+
 /// Read the recovery flag for `source_id`. A missing row OR an
 /// unparseable value reads as `true` ("clean") — the former covers the
 /// first-ever run on a virgin DB (no recovery needed), the latter