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/telegram.py

@@ -14,6 +14,25 @@ logger = logging.getLogger(__name__)
 _TELEGRAM_API = "https://api.telegram.org/bot{token}/{method}"
 
 
+async def validate_bot_token() -> bool:
+    """Validate BOT_TOKEN by calling getMe. Logs ERROR if invalid. Never raises."""
+    url = _TELEGRAM_API.format(token=config.BOT_TOKEN, method="getMe")
+    async with httpx.AsyncClient(timeout=10) as client:
+        try:
+            resp = await client.get(url)
+            if resp.status_code == 200:
+                bot_name = resp.json().get("result", {}).get("username", "?")
+                logger.info("Telegram token valid, bot: @%s", bot_name)
+                return True
+            logger.error(
+                "BOT_TOKEN invalid — getMe returned %s: %s", resp.status_code, resp.text
+            )
+            return False
+        except Exception as exc:
+            logger.error("BOT_TOKEN validation failed (network): %s", exc)
+            return False
+
+
 async def send_message(text: str) -> None:
     url = _TELEGRAM_API.format(token=config.BOT_TOKEN, method="sendMessage")
     async with httpx.AsyncClient(timeout=10) as client: