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>

backend/src/crawler/jobs.rs

@@ -1,4 +1,4 @@
-//! Persistent job queue and the four job kinds.
+//! Persistent job queue and its job kinds.
 //!
 //! Backed by Postgres (the `crawler_jobs` table). Workers lease rows
 //! with `SELECT ... FOR UPDATE SKIP LOCKED`, heartbeat via
@@ -12,16 +12,9 @@ use serde::{Deserialize, Serialize};
 use sqlx::PgPool;
 use uuid::Uuid;
 
-use super::source::DiscoverMode;
-
 #[derive(Clone, Debug, Serialize, Deserialize)]
 #[serde(tag = "kind", rename_all = "snake_case")]
 pub enum JobPayload {
-    /// Walk the source index and enqueue `SyncManga` jobs.
-    Discover {
-        source_id: String,
-        mode: DiscoverMode,
-    },
     /// Fetch one manga's detail page, upsert metadata, enqueue
     /// `SyncChapterList`.
     SyncManga {