fix(BATON-007): add validate_bot_token() for startup detection and fix test mocks

- Add validate_bot_token() to backend/telegram.py: calls getMe on startup,
  logs ERROR if token is invalid (never raises per #1215 contract)
- Call validate_bot_token() in lifespan() after db.init_db() for early detection
- Update conftest.py make_app_client() to mock getMe endpoint
- Add 3 tests for validate_bot_token (200, 401, network error cases)

Root cause: CHAT_ID=5190015988 (positive) was wrong — fixed to -5190015988
on server per decision #1212. Group "Big Red Button" confirmed via getChat.
Service restarted.

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

backend/main.py

@@ -71,6 +71,11 @@ async def lifespan(app: FastAPI):
     await db.init_db()
     logger.info("Database initialized")
 
+    if not await telegram.validate_bot_token():
+        logger.error(
+            "CRITICAL: BOT_TOKEN is invalid — Telegram delivery is broken. Update .env and restart."
+        )
+
     if config.WEBHOOK_ENABLED:
         await telegram.set_webhook(url=config.WEBHOOK_URL, secret=config.WEBHOOK_SECRET)
         logger.info("Webhook registered")