perf: denormalize content-warning filter to manga_content_warnings

The content-warning include/exclude filter (and its count) ran a correlated
page_content_warnings->pages->chapters join per candidate manga — O(mangas *
pages) on every filtered list. Add a manga_content_warnings(manga_id, warning)
table holding each manga's deduped warning union, maintained by triggers that
recompute an affected manga's set (delete+reinsert of the tiny five-label set,
robust across page/chapter/manga cascade deletes), backfilled in the migration.
The filter is now a single indexed lookup.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/tests/api_manga_warnings.rs

@@ -127,6 +127,46 @@ async fn list_filters_by_content_warning(pool: PgPool) {
     assert_eq!(ids, want);
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn denormalized_warnings_track_chapter_deletion(pool: PgPool) {
+    // The denormalized manga_content_warnings table must stay in sync when the
+    // underlying pages disappear. Deleting the only chapter (which cascade-
+    // deletes its pages and their page_content_warnings) must drop the manga
+    // from the include filter.
+    let h = common::harness(pool.clone());
+    let gory = seed_manga(&pool, "Gory", &[&["gore"]]).await;
+
+    // Present in the denorm table and matched by the filter.
+    let count: i64 =
+        sqlx::query_scalar("SELECT count(*) FROM manga_content_warnings WHERE manga_id = $1")
+            .bind(gory)
+            .fetch_one(&pool)
+            .await
+            .unwrap();
+    assert_eq!(count, 1, "warning denormalized on insert");
+    assert_eq!(list_ids(&h.app, "cw_include=gore").await, vec![gory.to_string()]);
+
+    // Delete the chapter → cascade removes pages + page_content_warnings →
+    // trigger recomputes the (now empty) set for this manga.
+    sqlx::query("DELETE FROM chapters WHERE manga_id = $1")
+        .bind(gory)
+        .execute(&pool)
+        .await
+        .unwrap();
+
+    let count: i64 =
+        sqlx::query_scalar("SELECT count(*) FROM manga_content_warnings WHERE manga_id = $1")
+            .bind(gory)
+            .fetch_one(&pool)
+            .await
+            .unwrap();
+    assert_eq!(count, 0, "warning removed after chapter (and pages) deleted");
+    assert!(
+        list_ids(&h.app, "cw_include=gore").await.is_empty(),
+        "manga no longer matches the include filter"
+    );
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn list_rejects_unknown_warning(pool: PgPool) {
     let h = common::harness(pool.clone());