feat(mangas): content-warning filter on search + deduped banner on detail

Surfaces the analysis worker's NSFW moderation at the manga level:

- repo::manga FILTER_WHERE gains cw_include (AND, page-content-warning
  join to mangas) and cw_exclude (NONE) clauses; ListQuery + binds
  renumbered ($6/$7, LIMIT/OFFSET $8/$9).
- repo::page_analysis::warnings_for_manga: deduped, alphabetical union
  across all the manga's pages.
- domain::MangaDetail gains content_warnings; get_detail populates it.
- api::mangas list accepts cw_include/cw_exclude (reusing the shared
  parse_warnings_csv validator).

Tests: detail union (deduped/sorted) + empty case; list include/exclude
filter; unknown-warning 422. Existing manga tests still green.

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

backend/src/repo/page_analysis.rs

@@ -77,6 +77,28 @@ pub async fn enqueue_all_pages(pool: &PgPool, only_unanalyzed: bool) -> AppResul
     Ok(result.rows_affected())
 }
 
+/// Deduplicated union of content warnings across all of a manga's pages,
+/// ordered alphabetically. Powers the manga-detail content-warning banner.
+pub async fn warnings_for_manga(
+    pool: &PgPool,
+    manga_id: uuid::Uuid,
+) -> AppResult<Vec<ContentWarning>> {
+    let rows = sqlx::query_scalar::<_, ContentWarning>(
+        r#"
+        SELECT DISTINCT pw.warning
+          FROM page_content_warnings pw
+          JOIN pages p    ON p.id = pw.page_id
+          JOIN chapters c ON c.id = p.chapter_id
+         WHERE c.manga_id = $1
+         ORDER BY pw.warning
+        "#,
+    )
+    .bind(manga_id)
+    .fetch_all(pool)
+    .await?;
+    Ok(rows)
+}
+
 /// Load a page's analysis row, if it has one.
 pub async fn load(pool: &PgPool, page_id: Uuid) -> AppResult<Option<PageAnalysis>> {
     let row = sqlx::query_as::<_, PageAnalysis>(