Add --check preflight to validate config and connectivity

`python -m gaming_deals_bot --check` tests each external dependency
before starting the bot:

- Matrix: verifies access token (whoami) and room membership
- CheapShark: hits the deals endpoint
- Epic Games Store: hits the free-games endpoint
- Frankfurter: fetches exchange rates
- IsThereAnyDeal: validates the API key (skipped if not set)

Exits 0/1 so it works in CI pipelines and Docker health-checks.

https://claude.ai/code/session_01LPpSZFfyh6vdV5HGFWjoQX

gaming_deals_bot/__main__.py

@@ -9,6 +9,7 @@ from apscheduler.schedulers.asyncio import AsyncIOScheduler
 
 from .bot import DealsBot
 from .config import Config
+from .preflight import run_preflight
 
 
 def setup_logging():
@@ -23,12 +24,18 @@ async def main():
     setup_logging()
     logger = logging.getLogger("gaming_deals_bot")
 
+    check_mode = "--check" in sys.argv
+
     try:
         config = Config()
     except ValueError as exc:
         logger.error("Configuration error: %s", exc)
         sys.exit(1)
 
+    if check_mode:
+        ok = await run_preflight(config)
+        sys.exit(0 if ok else 1)
+
     bot = DealsBot(config)
     await bot.start()