Perf: light worker poll query + tasks(status) index

Worker polls the DB every interval (production 30s) and every
poll ran SELECT * FROM tasks: full rows including result_text
and error_text (up to 4000 chars each) for every task ever,
growing with history. load_tasks now selects only the columns
pick_next actually needs (id/project/prompt/status/depends/
priority/timestamps/executor) and only queued/completed/merged
rows; running/failed rows are neither ready nor completed, so
excluding them cannot change pick_next. get_task still does
SELECT * for the single claimed task. Plus CREATE INDEX IF NOT
EXISTS idx_tasks_status on tasks(status) in both SQLite
init_db and the Postgres schema, so the poll filter stops being
a full table scan on old databases too. Test: load_tasks returns
only queued+completed out of queued/completed/running/failed.

cloud/tests/test_fallback_retry.py

@@ -384,6 +384,23 @@ class FallbackRetryTestCase(unittest.TestCase):
         self.assertEqual(len(self.calls), 1)
         self.assertEqual(result["events"], [])
 
+    def test_load_tasks_only_picks_relevant_statuses(self):
+        # load_tasks 只取 queued/completed/merged：running/failed
+        # 的任务对 pick_next 既不是就绪也不是已完成，排除它们
+        # 不影响结果，但省掉每轮 poll 全表 + 重字段的开销。
+        for tid, status in [("Q", "queued"), ("C", "completed"),
+                            ("R", "running"), ("F", "failed")]:
+            _insert_task(self.db_path, tid)
+            import sqlite3
+            conn = sqlite3.connect(self.db_path)
+            conn.execute(
+                "UPDATE tasks SET status = ? WHERE id = ?", (status, tid))
+            conn.commit()
+            conn.close()
+        worker = self._worker(runner=lambda cmd: None)
+        got = {t["id"] for t in worker.load_tasks()}
+        self.assertEqual(got, {"Q", "C"})
+
 
 if __name__ == "__main__":
     unittest.main()