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_jobs.rs

@@ -9,7 +9,6 @@ use std::time::Duration;
 use mangalord::crawler::jobs::{
     self, EnqueueResult, JobPayload, KIND_SYNC_CHAPTER_CONTENT,
 };
-use mangalord::crawler::source::DiscoverMode;
 use sqlx::PgPool;
 use uuid::Uuid;
 
@@ -21,10 +20,13 @@ fn chapter_content_payload(chapter_id: Uuid) -> JobPayload {
     }
 }
 
-fn discover_payload() -> JobPayload {
-    JobPayload::Discover {
+/// A non-`SyncChapterContent` payload, used to assert that only the
+/// chapter-content kind is deduplicated by the partial index and that
+/// `lease`'s kind filter correctly excludes other kinds.
+fn sync_manga_payload(key: &str) -> JobPayload {
+    JobPayload::SyncManga {
         source_id: "target".into(),
-        mode: DiscoverMode::Backfill,
+        source_manga_key: key.into(),
     }
 }
 
@@ -141,7 +143,7 @@ async fn different_chapter_ids_can_coexist(pool: PgPool) {
 
 #[sqlx::test(migrations = "./migrations")]
 async fn non_chapter_content_payloads_are_never_deduped(pool: PgPool) {
-    let p = discover_payload();
+    let p = sync_manga_payload("foo");
     assert!(matches!(
         jobs::enqueue(&pool, &p).await.unwrap(),
         EnqueueResult::Inserted(_)
@@ -185,7 +187,10 @@ async fn lease_marks_running_and_bumps_attempts_and_sets_leased_until(pool: PgPo
 
 #[sqlx::test(migrations = "./migrations")]
 async fn lease_with_kind_filter_only_matches_that_kind(pool: PgPool) {
-    let discover_id = match jobs::enqueue(&pool, &discover_payload()).await.unwrap() {
+    let manga_id = match jobs::enqueue(&pool, &sync_manga_payload("foo"))
+        .await
+        .unwrap()
+    {
         EnqueueResult::Inserted(id) => id,
         _ => unreachable!(),
     };
@@ -207,8 +212,8 @@ async fn lease_with_kind_filter_only_matches_that_kind(pool: PgPool) {
     .unwrap();
     assert_eq!(leases.len(), 1, "only chapter content payload leases");
     assert_eq!(leases[0].id, chapter_id);
-    // discover is still pending
-    assert_eq!(job_state(&pool, discover_id).await, "pending");
+    // sync_manga is still pending
+    assert_eq!(job_state(&pool, manga_id).await, "pending");
 }
 
 #[sqlx::test(migrations = "./migrations")]