fix(crawler): reap dead jobs alongside done ones

The retention reaper deleted only `state = 'done'` rows, so terminal
`dead` jobs (retries exhausted) accumulated in crawler_jobs forever —
slow unbounded table growth. Both states are end-of-life and never
leased again.

Rename `reap_done` → `reap_terminal` and broaden the filter to
`state IN ('done','dead')`; `pending`/`running` stay untouched. Test
extended to assert old done + old dead both reap while fresh terminal and
old-but-active rows survive.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/repo/crawler.rs

@@ -903,9 +903,8 @@ pub struct JobHistoryFilter<'a> {
 /// manga/chapter/page context (best-effort) so the table can label rows.
 /// Returns the page slice plus the filtered total for pagination.
 ///
-/// History depth is bounded by the done-job reaper (`reap_done`): completed
-/// jobs older than the retention window are gone. Terminal `dead` jobs
-/// persist until requeued.
+/// History depth is bounded by the terminal-job reaper (`reap_terminal`):
+/// `done` and `dead` jobs older than the retention window are gone.
 pub async fn list_job_history(
     pool: &PgPool,
     filter: JobHistoryFilter<'_>,