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/user.py

@@ -0,0 +1,31 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import Column, String, Boolean, DateTime
+from sqlalchemy.dialects.postgresql import UUID
+
+from app.database import Base
+
+
+class User(Base):
+    """
+    User model for authentication and authorization.
+    
+    Possible roles:
+    - admin: Full system access
+    - red_tech: Red team technician - can create and edit tests
+    - blue_tech: Blue team technician - can create and edit tests
+    - red_lead: Red team lead - can validate tests
+    - blue_lead: Blue team lead - can validate tests
+    - viewer: Read-only access (default)
+    """
+    __tablename__ = "users"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    username = Column(String, unique=True, nullable=False)
+    email = Column(String, nullable=True)
+    hashed_password = Column(String, nullable=False)
+    role = Column(String, nullable=False, default="viewer")
+    is_active = Column(Boolean, default=True)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    last_login = Column(DateTime, nullable=True)