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/test_telegram.py

@@ -34,11 +34,57 @@ import pytest
 import respx
 
 from backend import config
-from backend.telegram import SignalAggregator, send_message, set_webhook
+from backend.telegram import SignalAggregator, send_message, set_webhook, validate_bot_token
 
 
 SEND_URL = f"https://api.telegram.org/bot{config.BOT_TOKEN}/sendMessage"
 WEBHOOK_URL_API = f"https://api.telegram.org/bot{config.BOT_TOKEN}/setWebhook"
+GET_ME_URL = f"https://api.telegram.org/bot{config.BOT_TOKEN}/getMe"
+
+
+# ---------------------------------------------------------------------------
+# validate_bot_token
+# ---------------------------------------------------------------------------
+
+@pytest.mark.asyncio
+async def test_validate_bot_token_returns_true_on_200():
+    """validate_bot_token returns True when getMe responds 200."""
+    with respx.mock(assert_all_called=False) as mock:
+        mock.get(GET_ME_URL).mock(
+            return_value=httpx.Response(200, json={"ok": True, "result": {"username": "batonbot"}})
+        )
+        result = await validate_bot_token()
+    assert result is True
+
+
+@pytest.mark.asyncio
+async def test_validate_bot_token_returns_false_on_401(caplog):
+    """validate_bot_token returns False and logs ERROR when getMe responds 401."""
+    import logging
+
+    with respx.mock(assert_all_called=False) as mock:
+        mock.get(GET_ME_URL).mock(
+            return_value=httpx.Response(401, json={"ok": False, "description": "Unauthorized"})
+        )
+        with caplog.at_level(logging.ERROR, logger="backend.telegram"):
+            result = await validate_bot_token()
+
+    assert result is False
+    assert any("401" in record.message for record in caplog.records)
+
+
+@pytest.mark.asyncio
+async def test_validate_bot_token_returns_false_on_network_error(caplog):
+    """validate_bot_token returns False and logs ERROR on network failure — never raises."""
+    import logging
+
+    with respx.mock(assert_all_called=False) as mock:
+        mock.get(GET_ME_URL).mock(side_effect=httpx.ConnectError("connection refused"))
+        with caplog.at_level(logging.ERROR, logger="backend.telegram"):
+            result = await validate_bot_token()
+
+    assert result is False
+    assert len(caplog.records) >= 1
 
 
 # ---------------------------------------------------------------------------