feat(knowledge): 任务队列、独立 worker 与 pgvector 向量索引

知识处理改为数据库任务队列，独立 worker 做切块与向量化，失败可重试。

backend/app/knowledge/deadline.py

@@ -0,0 +1,39 @@
+"""
+Copyright 2024-2026 ChatterMate
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+How long one source may take to read.
+
+Reading runs on worker threads, which cannot be stopped from outside: a
+timeout around the call would mark the source failed while the crawl and
+its embedding carried on in the background. So the work checks the deadline
+itself, between pages and between chunks, and stops.
+"""
+
+import time
+from typing import Callable
+
+
+class SourceTimedOut(Exception):
+    """The source ran past its time limit. The message is for the user."""
+
+
+class Deadline:
+    def __init__(self, seconds: float, clock: Callable[[], float] = time.monotonic):
+        self._clock = clock
+        self._at = clock() + seconds
+
+    @property
+    def expired(self) -> bool:
+        return self._clock() >= self._at