fix: event-log route handles missing location_id

The /event-log route now shows a location selector dropdown instead of
returning a 422 error when no location_id is provided. This follows the
same pattern used by the Eggs page.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_web_events.py

@@ -107,10 +107,12 @@ def create_cohort(animal_service, location_id, count=3):
 class TestEventLogRoute:
     """Tests for GET /event-log route."""
 
-    def test_event_log_requires_location_id(self, client):
-        """Event log requires location_id parameter."""
+    def test_event_log_without_location_shows_selector(self, client):
+        """Event log without location_id shows location selector."""
         response = client.get("/event-log")
-        assert response.status_code == 422
+        assert response.status_code == 200
+        # Should show location selector prompt
+        assert "Select a location" in response.text
 
     def test_event_log_returns_empty_for_new_location(self, client, valid_location_id):
         """Event log returns empty state for location with no events."""