chore: drop dead 'failed' branch from crawler_jobs partial index

0012_crawler.sql's partial index on `state IN ('pending','failed')`
indexes a state that no code path ever writes — ack_failed in
crawler/jobs.rs only ever moves jobs to 'dead' or 'pending'. The
'failed' branch costs a write on every state change without ever
matching a query. Drop it; the CHECK still allows 'failed' so a
future migration can re-introduce it cleanly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/migrations/0016_crawler_jobs_drop_failed_state.sql

@@ -0,0 +1,15 @@
+-- The original 0012 partial index covers `state IN ('pending','failed')`,
+-- but `ack_failed` in src/crawler/jobs.rs only writes `dead` or
+-- `pending` — `failed` is never set. The index branch on `failed`
+-- never matches any row, so it's dead weight on every write.
+--
+-- Drop and recreate the index without the dead branch. The CHECK
+-- constraint on `state` still allows `'failed'` so a future migration
+-- can adopt that terminal-but-retryable state without a second
+-- schema change.
+
+DROP INDEX IF EXISTS crawler_jobs_ready_idx;
+
+CREATE INDEX crawler_jobs_ready_idx
+    ON crawler_jobs (scheduled_at)
+    WHERE state = 'pending';