db: enforce unique constraint on test_detection_results

- Add UniqueConstraint(test_id, detection_rule_id) named uq_tdr_test_rule to TestDetectionResult model

- Alembic b025: safely deduplicate existing rows before creating constraint

backend/app/models/test_detection_result.py

@@ -7,7 +7,7 @@ rule as triggered / not triggered / not applicable, along with notes.
 import uuid
 from datetime import datetime
 
-from sqlalchemy import Column, String, Text, Boolean, DateTime, ForeignKey, Index
+from sqlalchemy import Column, String, Text, Boolean, DateTime, ForeignKey, Index, UniqueConstraint
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import relationship
 
@@ -52,4 +52,5 @@ class TestDetectionResult(Base):
     __table_args__ = (
         Index('ix_tdr_test', 'test_id'),
         Index('ix_tdr_rule', 'detection_rule_id'),
+        UniqueConstraint('test_id', 'detection_rule_id', name='uq_tdr_test_rule'),
     )