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>

tests/conftest.py

@@ -79,6 +79,7 @@ def make_app_client():
     """
     tg_set_url = f"https://api.telegram.org/bot{config.BOT_TOKEN}/setWebhook"
     send_url = f"https://api.telegram.org/bot{config.BOT_TOKEN}/sendMessage"
+    get_me_url = f"https://api.telegram.org/bot{config.BOT_TOKEN}/getMe"
 
     @contextlib.asynccontextmanager
     async def _ctx():
@@ -86,6 +87,9 @@ def make_app_client():
             from backend.main import app
 
             mock_router = respx.mock(assert_all_called=False)
+            mock_router.get(get_me_url).mock(
+                return_value=httpx.Response(200, json={"ok": True, "result": {"username": "testbot"}})
+            )
             mock_router.post(tg_set_url).mock(
                 return_value=httpx.Response(200, json={"ok": True, "result": True})
             )