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/domain/manga.rs

@@ -5,6 +5,7 @@ use uuid::Uuid;
 
 use super::author::AuthorRef;
 use super::genre::GenreRef;
+use super::page_analysis::ContentWarning;
 use super::patch::Patch;
 use super::tag::TagRef;
 
@@ -32,7 +33,8 @@ pub struct MangaCard {
 }
 
 /// Shape returned by `GET /mangas/:id`. Adds user-added tags on top of
-/// the card fields.
+/// the card fields, plus the deduped content warnings derived by the
+/// analysis worker across all the manga's pages.
 #[derive(Debug, Clone, Serialize)]
 pub struct MangaDetail {
     #[serde(flatten)]
@@ -40,6 +42,7 @@ pub struct MangaDetail {
     pub authors: Vec<AuthorRef>,
     pub genres: Vec<GenreRef>,
     pub tags: Vec<TagRef>,
+    pub content_warnings: Vec<ContentWarning>,
 }
 
 #[derive(Debug, Clone, Deserialize, Default)]