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/pipeline.rs

@@ -427,11 +427,7 @@ async fn download_and_store_cover(
     Ok(())
 }
 
-fn origin_of(url: &str) -> Option<String> {
-    let (scheme, rest) = url.split_once("://")?;
-    let host = rest.split('/').next()?;
-    Some(format!("{scheme}://{host}"))
-}
+use crate::crawler::url_utils::origin_of;
 
 #[cfg(test)]
 mod tests {