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/models/enums.py

@@ -0,0 +1,23 @@
+import enum
+
+
+class TechniqueStatus(str, enum.Enum):
+    not_evaluated = "not_evaluated"
+    in_progress = "in_progress"
+    validated = "validated"
+    partial = "partial"
+    not_covered = "not_covered"
+    review_required = "review_required"
+
+
+class TestState(str, enum.Enum):
+    draft = "draft"
+    in_review = "in_review"
+    validated = "validated"
+    rejected = "rejected"
+
+
+class TestResult(str, enum.Enum):
+    detected = "detected"
+    not_detected = "not_detected"
+    partially_detected = "partially_detected"