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/technique.py

@@ -0,0 +1,39 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import Column, String, Text, Boolean, DateTime, Enum
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+from sqlalchemy.orm import relationship
+
+from app.database import Base
+from app.models.enums import TechniqueStatus
+
+
+class Technique(Base):
+    """
+    MITRE ATT&CK Technique model.
+    
+    Represents an attack technique from the MITRE ATT&CK framework,
+    including its coverage status and associated tests.
+    """
+    __tablename__ = "techniques"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    mitre_id = Column(String, unique=True, nullable=False)  # e.g., "T1059.001"
+    name = Column(String, nullable=False)
+    description = Column(Text, nullable=True)
+    tactic = Column(String, nullable=True)
+    platforms = Column(JSONB, nullable=True, default=[])
+    mitre_version = Column(String, nullable=True)
+    mitre_last_modified = Column(DateTime, nullable=True)
+    is_subtechnique = Column(Boolean, default=False)
+    parent_mitre_id = Column(String, nullable=True)
+    status_global = Column(
+        Enum(TechniqueStatus, name="techniquestatus"),
+        default=TechniqueStatus.not_evaluated
+    )
+    review_required = Column(Boolean, default=False)
+    last_review_date = Column(DateTime, nullable=True)
+
+    # Relationships
+    tests = relationship("Test", back_populates="technique")