fix(server): reject inverted period_start/period_end in CameraRecord

A misbehaving or clock-broken device could submit period_end <=
period_start, polluting the camera_records table with zero-length or
inverted windows that corrupt downstream hourly analytics. Add a
Pydantic model_validator so the request is rejected at the API
boundary instead of silently persisting bad ranges.

Found via adversarial review (run 2026-05-01-191359, both reviewers).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/test_camera_endpoint.py

@@ -98,3 +98,15 @@ def test_negative_counts_rejected():
     with pytest.raises(ValidationError):
         CameraRecord(period_start=1712000000, period_end=1712003600,
                      entries=-1, exits=0)
+
+
+def test_inverted_period_rejected():
+    """Pydantic should reject period_end <= period_start."""
+    from pydantic import ValidationError
+    from server.camera_endpoint import CameraRecord
+    with pytest.raises(ValidationError):
+        CameraRecord(period_start=1712003600, period_end=1712003600,
+                     entries=0, exits=0)
+    with pytest.raises(ValidationError):
+        CameraRecord(period_start=1712003600, period_end=1712000000,
+                     entries=0, exits=0)