feat: transient-page detection across the crawler (0.30.0)

Until now, when the target site returned its 403 "we're sorry, the
request file are not found" response on a page that actually exists,
selectors matched nothing and the crawler treated the page as
"legitimately empty". Pagination walks silently dropped whole pages
worth of mangas, fetch_manga skipped individual entries, and the
startup session probe blamed PHPSESSID for what was a site hiccup.

This branch adds a single detection layer that the whole pipeline
routes through:

- `crawler::detect`: PageError::Transient typed signal, plus two
  primitives (`is_broken_page_body` matches the universal 403 body;
  `has_logo_sentinel` asserts #logo, the site-wide header element)
  and a `retry_on_transient` helper that retries a closure on
  Transient with a small attempt budget.
- `navigate()` screens every fetched body for the broken-page
  signature before handing it to a selector.
- Parsers (`parse_manga_list_from`, `parse_manga_detail`,
  `parse_chapter_pages`) check their structural sentinels (#logo for
  full-layout pages; a#pic_container for the reader, which doesn't
  render #logo) and return Result<_, PageError>. Empty Vec is now
  reserved for genuinely empty pages.
- `discover()` retries each pagination page up to 3× (2s apart) before
  failing the whole Discover job — at which point the existing job
  system's retry/backoff takes over for longer outages.
- `verify_session` is three-state: broken-page → retry probe;
  #logo present but #avatar_menu absent → genuine logout (the only
  state that should blame PHPSESSID); both present → ok.

Test coverage added at the helper level: 13 unit tests for the
detection module (body signature, logo sentinel, PageError, retry
helper), parser-level tests for both transient and legitimately-empty
inputs, and 6 unit tests for the session probe classifier.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.29.0",
+  "version": "0.30.0",
   "private": true,
   "type": "module",
   "scripts": {