test(kevin): fix enum assertion + mark Postgres-dependent tests as integration

Pipeline #46 surfaced two pre-existing CI bugs once fakeredis was
installed and tests could collect:

1. test_models.py:389 asserted "DISCOVERED" in status_col.type.enums,
   but the model defines KevinVideoStatus with values_callable so
   .enums returns the lowercase string values, not member names.
   Asserting "discovered" instead.

2. Four test files use the db_session fixture which requires a real
   Postgres on localhost:5432. CI has no Postgres, so 10 tests failed
   with Connect call failed (errno 111). These genuinely need a DB —
   mirroring tests/integration/* which already use
   @pytest.mark.integration. Adding module-level
   pytestmark = pytest.mark.integration to:
   - tests/shared/models/test_meet_kevin_trading.py
   - tests/services/kevin_signal_bridge/test_aggregator.py
   - tests/services/kevin_signal_bridge/test_audit.py
   - tests/services/kevin_signal_bridge/test_exit_scanner.py

CI runs with -m "not integration" so they're now deselected.
Local pytest still picks them up by default (no marker filter).

tests/shared/models/test_meet_kevin_trading.py

@@ -8,6 +8,8 @@ import pytest
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
+pytestmark = pytest.mark.integration
+
 from shared.models.meet_kevin import (
     KevinAnalysis,
     KevinChannel,