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/crawler/daemon.rs

@@ -317,14 +317,10 @@ impl WorkerContext {
         // (because a force-refetch race or a job that was re-enqueued
         // after a previous one finished), ack done without re-fetching.
         if let JobPayload::SyncChapterContent { chapter_id, .. } = &lease.payload {
-            let page_count: Option<i32> = sqlx::query_scalar(
-                "SELECT page_count FROM chapters WHERE id = $1",
-            )
-            .bind(chapter_id)
-            .fetch_optional(&self.pool)
-            .await
-            .ok()
-            .flatten();
+            let page_count = crate::repo::chapter::page_count(&self.pool, *chapter_id)
+                .await
+                .ok()
+                .flatten();
             if matches!(page_count, Some(n) if n > 0) {
                 let _ = jobs::ack_done(&self.pool, lease.id).await;
                 return;