fix: 将 RAG 表 ID 字段从 VARCHAR(32) 改为 VARCHAR(36)

修复 UUID 带连字符格式（36字符）超出 VARCHAR(32) 限制的问题

backend/sql/rag_tables_compatible.sql

@@ -11,9 +11,9 @@ CREATE EXTENSION IF NOT EXISTS vector;
 -- 1. 文本分块表 (text_chunks)
 -- ============================================
 CREATE TABLE IF NOT EXISTS text_chunks (
-    id VARCHAR(32) PRIMARY KEY,
-    document_id VARCHAR(32) NOT NULL,
-    text_storage_id VARCHAR(32),
+    id VARCHAR(36) PRIMARY KEY,
+    document_id VARCHAR(36) NOT NULL,
+    text_storage_id VARCHAR(36),
     chunk_index INTEGER NOT NULL,
     content TEXT NOT NULL,
     token_count INTEGER,
@@ -35,8 +35,8 @@ CREATE INDEX IF NOT EXISTS idx_text_chunks_chunk_index ON text_chunks(document_i
 -- 2. 向量嵌入表 (vector_embeddings)
 -- ============================================
 CREATE TABLE IF NOT EXISTS vector_embeddings (
-    id VARCHAR(32) PRIMARY KEY,
-    chunk_id VARCHAR(32) NOT NULL REFERENCES text_chunks(id) ON DELETE CASCADE,
+    id VARCHAR(36) PRIMARY KEY,
+    chunk_id VARCHAR(36) NOT NULL REFERENCES text_chunks(id) ON DELETE CASCADE,
     embedding vector(768),  -- nomic-embed-text 维度为 768
     model_name VARCHAR(100) DEFAULT 'nomic-embed-text',
     create_time TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
@@ -58,12 +58,12 @@ CREATE INDEX IF NOT EXISTS idx_vector_embeddings_hnsw ON vector_embeddings
 -- 按文档ID检索相似文本块
 CREATE OR REPLACE FUNCTION search_similar_chunks(
     query_embedding vector(768),
-    target_document_id VARCHAR(32),
+    target_document_id VARCHAR(36),
     result_limit INTEGER DEFAULT 3
 )
 RETURNS TABLE (
-    chunk_id VARCHAR(32),
-    document_id VARCHAR(32),
+    chunk_id VARCHAR(36),
+    document_id VARCHAR(36),
     content TEXT,
     chunk_index INTEGER,
     similarity FLOAT
@@ -90,8 +90,8 @@ CREATE OR REPLACE FUNCTION search_similar_chunks_global(
     result_limit INTEGER DEFAULT 5
 )
 RETURNS TABLE (
-    chunk_id VARCHAR(32),
-    document_id VARCHAR(32),
+    chunk_id VARCHAR(36),
+    document_id VARCHAR(36),
     content TEXT,
     chunk_index INTEGER,
     similarity FLOAT