feat: auto-migrate SQLite data to Postgres on first deploy

- Add trumpsignal.db to git (one-time migration source)
- entrypoint.sh detects empty Postgres and runs migration automatically
- scripts/migrate_sqlite_to_postgres.py: posts, subscriptions, bot_trades
- Add psycopg2-binary to requirements for migration script

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -7,8 +7,7 @@ env/
 .env
 *.pem
 
-# 数据库
-*.db
+# 数据库 (trumpsignal.db 是例外，用于一次性迁移到 Postgres)
 *.sqlite
 *.sqlite3