feat(crawler): single-mode walker gated by recovery flag (0.36.0)

Collapses the crawler to a single newest-first walker and replaces the
N-consecutive-unchanged streak with a per-manga rule: stop on the first
manga where metadata is Unchanged AND chapter sync reports zero new
chapters. The early stop is gated by a per-source recovery flag stored
in `crawler_state` — set to `false` when a run starts, back to `true`
only on a clean exit (end-of-walk or intentional stop). A crashed run
leaves the flag `false` automatically (no shutdown code runs), so the
next tick walks the full catalog instead of bailing at the first
caught-up manga.

This means a crashed mid-walk run self-heals on the next tick: the
flag stays `false`, the next walk visits every page (recovering
anything the crash missed past its crash point), and steady state
resumes once the recovery sweep reaches end-of-walk.

Removed:
- DiscoverMode enum, Backfill mode, the boundary re-check +
  displaced-refs machinery in TargetSourceWalker.
- Drop-pass (mark_dropped_mangas) and seed-completion plumbing
  (mark_seed_completed / seed_completed_at). The recovery flag
  subsumes the seed-completion signal; drop detection was explicitly
  opted out.
- JobPayload::Discover (no production callers).
- CRAWLER_MODE / CRAWLER_INCREMENTAL_STOP_AFTER env vars and the
  CrawlerModePref config type.

`should_mark_clean_exit(walked_to_completion, hit_stop_condition)`
encodes the clean-exit truth table in its signature — `hit_limit` is
deliberately absent so a future edit cannot accidentally count a
caller-imposed cap as a clean exit.

Net -501 lines, 261 backend tests passing.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.35.6",
+  "version": "0.36.0",
   "private": true,
   "type": "module",
   "scripts": {