feat: crawler manga-list & metadata sync with cover download (0.23.0)

- TargetSource: first concrete impl of the Source trait, modeled on
  the old Puppeteer crawler's selectors (+ status normalization,
  tag-count stripping, chapter list)
- DiscoverMode::Backfill walks pagination last->1, reverse within each
  page (oldest-first); Incremental walks forward
- RateLimiter (tokio-time aware) plumbed through FetchContext so the
  pagination walk honors the same per-host budget as the outer loop
- repo::crawler: ensure_source, upsert_manga_from_source (returns
  New/Updated/Unchanged + current cover_image_path for backfill
  decisions), sync_manga_chapters, mark_dropped_mangas — all
  transactional, with case-insensitive lookups and source-insertable
  genres
- Cover image download via reqwest+infer; stored under
  mangas/{id}/cover.{ext} via the Storage trait
- Single CRAWLER_PROXY env wires both Chromium (--proxy-server) and
  reqwest::Proxy::all (HTTP/HTTPS/SOCKS5)
- Crawler binary: positional start URL or $CRAWLER_START_URL,
  $CRAWLER_LIMIT (cap fetches + skip drop pass on partial runs),
  $CRAWLER_SKIP_CHAPTERS (disable selector AND sync), $CRAWLER_RATE_MS
- Silences chromiumoxide 0.7's known CDP deserialize log spam via
  default tracing filter + CdpError::Serde downgrade
- 9 sqlx integration tests + 11 selector/rate-limit unit tests

backend/src/crawler/mod.rs

@@ -16,4 +16,5 @@
 pub mod browser;
 pub mod diff;
 pub mod jobs;
+pub mod rate_limit;
 pub mod source;