feat: add conversation_tables migration

Add migration for conversation_tables to support dynamic per-user conversation storage.

- CREATE TABLE conversation_tables with telegram_id, table_name, message_count
- Add indexes on last_message_at and message_count for query performance
- Tested successfully on local D1 database

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

migrations/008_conversation_tables.sql

@@ -0,0 +1,14 @@
+-- 대화 테이블 메타 정보
+-- 사용자별 동적 대화 테이블 관리
+
+CREATE TABLE IF NOT EXISTS conversation_tables (
+  telegram_id TEXT PRIMARY KEY,
+  table_name TEXT NOT NULL,
+  message_count INTEGER DEFAULT 0,
+  last_message_at DATETIME,
+  created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+-- 인덱스
+CREATE INDEX IF NOT EXISTS idx_conv_tables_last_msg ON conversation_tables(last_message_at DESC);
+CREATE INDEX IF NOT EXISTS idx_conv_tables_count ON conversation_tables(message_count DESC);