feat: migrate existing message_buffer data to conversation tables

Migrated 2 users:
- 7265981403: 18 messages
- 821596605: 46 messages

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/009_migrate_existing_conversations.sql

@@ -0,0 +1,61 @@
+-- 기존 message_buffer 데이터를 새 conversation 테이블로 마이그레이션
+-- 대상: 2명 사용자 (telegram_id: 7265981403, 821596605)
+
+-- 1. 사용자 7265981403 테이블 생성
+CREATE TABLE IF NOT EXISTS conv_7265981403 (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  role TEXT NOT NULL CHECK(role IN ('user', 'assistant')),
+  content TEXT NOT NULL,
+  tool_calls TEXT,
+  tool_results TEXT,
+  created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX IF NOT EXISTS idx_conv_7265981403_created ON conv_7265981403(created_at DESC);
+
+-- 2. 사용자 821596605 테이블 생성
+CREATE TABLE IF NOT EXISTS conv_821596605 (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  role TEXT NOT NULL CHECK(role IN ('user', 'assistant')),
+  content TEXT NOT NULL,
+  tool_calls TEXT,
+  tool_results TEXT,
+  created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX IF NOT EXISTS idx_conv_821596605_created ON conv_821596605(created_at DESC);
+
+-- 3. 사용자 7265981403 메시지 복사 (role: bot -> assistant)
+INSERT INTO conv_7265981403 (role, content, created_at)
+SELECT
+  CASE WHEN role = 'bot' THEN 'assistant' ELSE 'user' END,
+  message,
+  created_at
+FROM message_buffer
+WHERE user_id = 3
+ORDER BY created_at ASC;
+
+-- 4. 사용자 821596605 메시지 복사
+INSERT INTO conv_821596605 (role, content, created_at)
+SELECT
+  CASE WHEN role = 'bot' THEN 'assistant' ELSE 'user' END,
+  message,
+  created_at
+FROM message_buffer
+WHERE user_id = 1
+ORDER BY created_at ASC;
+
+-- 5. conversation_tables 메타 테이블에 등록
+INSERT OR REPLACE INTO conversation_tables (telegram_id, table_name, message_count, last_message_at)
+SELECT
+  '7265981403',
+  'conv_7265981403',
+  (SELECT COUNT(*) FROM conv_7265981403),
+  (SELECT MAX(created_at) FROM conv_7265981403);
+
+INSERT OR REPLACE INTO conversation_tables (telegram_id, table_name, message_count, last_message_at)
+SELECT
+  '821596605',
+  'conv_821596605',
+  (SELECT COUNT(*) FROM conv_821596605),
+  (SELECT MAX(created_at) FROM conv_821596605);