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/worker.py

@@ -365,10 +365,26 @@ class Worker:
 
     # -- DB 读写 ------------------------------------------------------
     def load_tasks(self):
+        """pick_next 的输入：只取 queued/completed/merged 三类任务。
+
+        旧实现是 SELECT * 全表：prompt/result_text/error_text
+        （后两者各可达 4000 字）全量进内存，且每轮 poll 都来一次；
+        任务表随时间变大后这是纯浪费。pick_next 只需要 queued
+        任务和已完成 id（依赖判断），running/failed/interrupted
+        的任务对它既不是就绪、也不是已完成，排除它们不影响结果。
+        列清单必须覆盖 _task_from_row 直接取的全部字段（prompt、
+        idempotency_key 等），缺一列就会 KeyError；result_text、
+        error_text 等重字段不在此列，真正需要它们的 get_task
+        仍是 SELECT * 单行取。
+        """
         conn = _db_connect(self.db_path)
         try:
             rows = conn.execute(
-                "SELECT * FROM tasks ORDER BY created_at, id").fetchall()
+                "SELECT id, project_id, prompt, status, idempotency_key,"
+                " depends_on, priority, created_by, created_at, updated_at,"
+                " last_heartbeat, executor FROM tasks"
+                " WHERE status IN ('queued', 'completed', 'merged')"
+                " ORDER BY created_at, id").fetchall()
         finally:
             conn.close()
         return [_task_from_row(r) for r in rows]