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/__init__.py

@@ -0,0 +1,13 @@
+# Import all models here so Alembic can detect them
+from app.models.user import User
+from app.models.technique import Technique
+from app.models.test import Test
+from app.models.evidence import Evidence
+from app.models.intel import IntelItem
+from app.models.audit import AuditLog
+from app.models.enums import TechniqueStatus, TestState, TestResult
+
+__all__ = [
+    "User", "Technique", "Test", "Evidence", "IntelItem", "AuditLog",
+    "TechniqueStatus", "TestState", "TestResult"
+]