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/crawl_metrics.rs

@@ -187,7 +187,7 @@ pub async fn list_ops(
 }
 
 /// Delete metric rows older than `retention_days`. `0` disables the reaper
-/// (returns 0 without touching the table). Mirrors `jobs::reap_done`.
+/// (returns 0 without touching the table). Mirrors `jobs::reap_terminal`.
 pub async fn reap(pool: &PgPool, retention_days: u32) -> sqlx::Result<u64> {
     if retention_days == 0 {
         return Ok(0);