refactor: remove dead code (safe cleanup)

- main.py: remove duplicate sqlalchemy select import
- main.py: remove unused json import in _telegram_poll_loop
- runtime.py: remove unreachable return after raise LockTimeoutError
- runtime.py: remove _restart_rdp_slot_bg (defined but never called)

app/main.py

@@ -13,7 +13,6 @@ from fastapi.responses import HTMLResponse, JSONResponse, RedirectResponse
 from fastapi.staticfiles import StaticFiles
 from fastapi.templating import Jinja2Templates
 from markupsafe import Markup, escape
-from sqlalchemy import select
 from sqlalchemy import delete, select, text, update
 from sqlalchemy.orm import Session
 from starlette.responses import HTMLResponse as _HR
@@ -151,7 +150,6 @@ _tg_poll_lock_file = None
 async def _telegram_poll_loop():
     """Poll Telegram for callback_query updates every 5 minutes."""
     import asyncio as _asyncio
-    import json as _jp
     global _tg_poll_offset
     await _asyncio.sleep(10)  # wait for app to fully start
     while True: