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/tests/crawler_sync.rs

@@ -528,62 +528,6 @@ async fn sync_chapters_serializes_concurrent_calls_for_same_manga(pool: PgPool)
     );
 }
 
-#[sqlx::test(migrations = "./migrations")]
-async fn mark_dropped_mangas_only_drops_unseen(pool: PgPool) {
-    crawler::ensure_source(&pool, "target", "T", "https://x.example")
-        .await
-        .unwrap();
-    // Seed two mangas before "now" so a later run_started_at sees them as stale.
-    let _ = crawler::upsert_manga_from_source(
-        &pool,
-        "target",
-        "https://x.example/foo",
-        &sample_manga("foo", "Foo", "hf"),
-    )
-    .await
-    .unwrap();
-    let _ = crawler::upsert_manga_from_source(
-        &pool,
-        "target",
-        "https://x.example/bar",
-        &sample_manga("bar", "Bar", "hb"),
-    )
-    .await
-    .unwrap();
-
-    // Now mark a new "run" beginning. Re-upsert only `foo` — `bar`
-    // should be the one flagged dropped.
-    let run_started = chrono::Utc::now();
-    // Sleep briefly so the second upsert's NOW() > run_started_at.
-    tokio::time::sleep(std::time::Duration::from_millis(20)).await;
-    let _ = crawler::upsert_manga_from_source(
-        &pool,
-        "target",
-        "https://x.example/foo",
-        &sample_manga("foo", "Foo", "hf"),
-    )
-    .await
-    .unwrap();
-
-    let n = crawler::mark_dropped_mangas(&pool, "target", run_started)
-        .await
-        .unwrap();
-    assert_eq!(n, 1, "only bar should have been dropped");
-
-    let foo_dropped: (Option<chrono::DateTime<chrono::Utc>>,) =
-        sqlx::query_as("SELECT dropped_at FROM manga_sources WHERE source_manga_key = 'foo'")
-            .fetch_one(&pool)
-            .await
-            .unwrap();
-    assert!(foo_dropped.0.is_none(), "foo seen this run, must not be dropped");
-    let bar_dropped: (Option<chrono::DateTime<chrono::Utc>>,) =
-        sqlx::query_as("SELECT dropped_at FROM manga_sources WHERE source_manga_key = 'bar'")
-            .fetch_one(&pool)
-            .await
-            .unwrap();
-    assert!(bar_dropped.0.is_some());
-}
-
 #[sqlx::test(migrations = "./migrations")]
 async fn upsert_surfaces_cover_image_path_for_backfill_decisions(pool: PgPool) {
     crawler::ensure_source(&pool, "target", "T", "https://x.example")