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/scripts/init-vector.sql

@@ -0,0 +1,19 @@
+-- Enable the vector extension
+CREATE EXTENSION IF NOT EXISTS vector;
+
+-- Create the ai schema if it doesn't exist
+CREATE SCHEMA IF NOT EXISTS ai;
+
+-- Set up vector similarity search functions
+CREATE OR REPLACE FUNCTION vector_similarity(vector1 vector, vector2 vector)
+RETURNS float
+AS $$
+BEGIN
+    RETURN 1 - (vector1 <=> vector2);
+END;
+$$ LANGUAGE plpgsql;
+
+-- Ensure proper permissions
+GRANT USAGE ON SCHEMA ai TO postgres;
+GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA ai TO postgres;
+GRANT ALL PRIVILEGES ON ALL SEQUENCES IN SCHEMA ai TO postgres;