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/api/mangas.rs

@@ -41,6 +41,12 @@ pub struct ListParams {
     pub genre_id: Option<String>,
     #[serde(default)]
     pub tag_id: Option<String>,
+    /// Comma-separated content warnings the manga must carry (AND).
+    #[serde(default)]
+    pub cw_include: Option<String>,
+    /// Comma-separated content warnings the manga must NOT carry (any).
+    #[serde(default)]
+    pub cw_exclude: Option<String>,
     #[serde(default = "default_limit")]
     pub limit: i64,
     #[serde(default)]
@@ -94,6 +100,8 @@ async fn list(
         author_ids: parse_uuid_csv("author_id", params.author_id.as_deref())?,
         genre_ids: parse_uuid_csv("genre_id", params.genre_id.as_deref())?,
         tag_ids: parse_uuid_csv("tag_id", params.tag_id.as_deref())?,
+        cw_include: crate::api::page_tags::parse_warnings_csv(params.cw_include.as_deref())?,
+        cw_exclude: crate::api::page_tags::parse_warnings_csv(params.cw_exclude.as_deref())?,
         limit,
         offset,
         sort: params.sort,