feat: Phase 1 - Data models and migrations (T-004 to T-009)

Implements all database models for the Aegis platform with full
Alembic migration support.

Models created:
- User: Authentication with role-based access control
- Technique: MITRE ATT&CK techniques with coverage status tracking
- Test: Security tests with validation workflow (draft/review/validated)
- Evidence: File metadata for test evidence (stored in MinIO)
- IntelItem: Threat intelligence items linked to techniques
- AuditLog: System-wide audit trail with JSONB details

Enumerations:
- TechniqueStatus: not_evaluated, in_progress, validated, partial, etc.
- TestState: draft, in_review, validated, rejected
- TestResult: detected, not_detected, partially_detected

Services:
- audit_service.py: log_action() helper for audit logging

All models include proper foreign key relationships and PostgreSQL
enum types are managed correctly in migrations (create/drop).

backend/app/services/audit_service.py

@@ -0,0 +1,33 @@
+from sqlalchemy.orm import Session
+
+from app.models.audit import AuditLog
+
+
+def log_action(
+    db: Session,
+    user_id,
+    action: str,
+    entity_type: str = None,
+    entity_id: str = None,
+    details: dict = None
+):
+    """
+    Log an action to the audit log.
+    
+    Args:
+        db: Database session
+        user_id: UUID of the user performing the action (can be None for system actions)
+        action: Description of the action (e.g., "create_test", "validate_technique")
+        entity_type: Type of entity affected (e.g., "technique", "test", "user")
+        entity_id: ID of the entity affected
+        details: Additional details as a dictionary (stored as JSONB)
+    """
+    log = AuditLog(
+        user_id=user_id,
+        action=action,
+        entity_type=entity_type,
+        entity_id=str(entity_id) if entity_id else None,
+        details=details,
+    )
+    db.add(log)
+    db.commit()