chore: dedupe is_unique_violation, lift SQL into repo, centralise URL parsing

Three layering cleanups from REVIEW.md §5 / §3:

- Drop the three private `is_unique_violation` helpers in
  repo::{user,chapter,bookmark} in favour of sqlx 0.8's
  `DatabaseError::is_unique_violation()` method (already used by
  repo::collection).
- Remove the unreachable 23505 branch in repo::chapter::create — the
  (manga_id, number) UNIQUE was dropped in 0013, so the defensive arm
  could no longer fire. A doc note records what to do if uniqueness
  is re-added.
- Move three inline SQL queries out of handlers/daemon into repo
  functions: bookmarks' chapter-belongs-to-manga guard
  (`repo::chapter::belongs_to_manga`), the daemon's dispatch lookup
  (`repo::chapter::dispatch_target`), and the daemon's page_count
  safety net (`repo::chapter::page_count`). Restores the
  handlers→repo layering invariant in CLAUDE.md.
- New `crawler::url_utils` module consolidates host_of / origin_of /
  registrable_domain — they used to live in three crawler submodules
  with diverging edge-case behaviour. Tests moved with them.
- Doc cross-references on repo::author::set_for_manga and
  repo::genre::set_for_manga pointing to the crawler's name-keyed
  variants, so the intentional duplication is discoverable.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/api/bookmarks.rs

@@ -67,14 +67,7 @@ async fn create(
     // the foreign-key violation collapse into a generic 500.
     repo::manga::get(&state.db, input.manga_id).await?;
     if let Some(chapter_id) = input.chapter_id {
-        let exists: Option<(Uuid,)> = sqlx::query_as(
-            "SELECT id FROM chapters WHERE id = $1 AND manga_id = $2",
-        )
-        .bind(chapter_id)
-        .bind(input.manga_id)
-        .fetch_optional(&state.db)
-        .await?;
-        if exists.is_none() {
+        if !repo::chapter::belongs_to_manga(&state.db, chapter_id, input.manga_id).await? {
             return Err(AppError::NotFound);
         }
     }