Add AI job queue indexes

internal/migrate/sql/004_ai_jobs_queue_indexes.up.sql

@@ -0,0 +1,13 @@
+CREATE INDEX IF NOT EXISTS ai_jobs_pending_claim_scope_idx
+    ON ai_jobs (task_type, model_profile, priority DESC, scheduled_at DESC, created_at DESC)
+    WHERE status = 'pending';
+
+CREATE INDEX IF NOT EXISTS ai_jobs_running_lease_idx
+    ON ai_jobs ((COALESCE(heartbeat_at, started_at, updated_at)))
+    WHERE status = 'running';
+
+CREATE INDEX IF NOT EXISTS ai_jobs_queue_stats_idx
+    ON ai_jobs (task_type, model_profile, status);
+
+CREATE INDEX IF NOT EXISTS ai_jobs_owner_stats_idx
+    ON ai_jobs (owner_service, task_type, model_profile, status);