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/crawler_identity.py

@@ -0,0 +1,22 @@
+"""
+Copyright 2024-2026 ReplyPier
+
+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.
+
+Who our crawler says it is. Honest, and the same for every request: hosts
+such as WordPress.com refuse the stale "Chrome 120 on Windows" string
+scrapers commonly fake, while a named crawler is let through - and site
+owners can see (and allow or block) who is reading them.
+"""
+
+CRAWLER_USER_AGENT = "Mozilla/5.0 (compatible; ReplyPierBot/1.0; +https://replypier.site)"