feat: add river-backed async task queue

apps/api/migrations/0032_river_async_queue.sql

@@ -0,0 +1,19 @@
+ALTER TABLE IF EXISTS gateway_tasks
+  ADD COLUMN IF NOT EXISTS river_job_id bigint,
+  ADD COLUMN IF NOT EXISTS remote_task_id text,
+  ADD COLUMN IF NOT EXISTS remote_task_payload jsonb;
+
+UPDATE gateway_tasks
+SET remote_task_payload = '{}'::jsonb
+WHERE remote_task_payload IS NULL;
+
+ALTER TABLE IF EXISTS gateway_tasks
+  ALTER COLUMN remote_task_payload SET DEFAULT '{}'::jsonb;
+
+CREATE INDEX IF NOT EXISTS idx_gateway_tasks_river_job
+  ON gateway_tasks(river_job_id)
+  WHERE river_job_id IS NOT NULL;
+
+CREATE INDEX IF NOT EXISTS idx_gateway_tasks_async_recover
+  ON gateway_tasks(async_mode, status, priority, created_at)
+  WHERE async_mode = true AND status IN ('queued', 'running');