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/app.rs

@@ -304,18 +304,9 @@ impl ChapterDispatcher for RealChapterDispatcher {
                 chapter_id,
                 source_chapter_key: _,
             } => {
-                // Look up manga_id + source_url for this chapter.
-                let row: Option<(uuid::Uuid, String)> = sqlx::query_as(
-                    "SELECT c.manga_id, cs.source_url \
-                       FROM chapters c \
-                       JOIN chapter_sources cs ON cs.chapter_id = c.id \
-                      WHERE c.id = $1 \
-                      LIMIT 1",
-                )
-                .bind(chapter_id)
-                .fetch_optional(&self.db)
-                .await
-                .context("look up chapter for dispatch")?;
+                let row = repo::chapter::dispatch_target(&self.db, chapter_id)
+                    .await
+                    .context("look up chapter for dispatch")?;
                 let Some((manga_id, source_url)) = row else {
                     // Chapter (or its source row) is gone — ack done.
                     return Ok(SyncOutcome::Skipped);