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/test.py

@@ -0,0 +1,40 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import Column, String, Text, DateTime, ForeignKey, Enum
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import relationship
+
+from app.database import Base
+from app.models.enums import TestState, TestResult
+
+
+class Test(Base):
+    """
+    Test model representing a security test for a MITRE ATT&CK technique.
+    
+    Each test documents an attempt to validate coverage of a specific technique,
+    including the procedure, tools used, and outcome.
+    """
+    __tablename__ = "tests"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    technique_id = Column(UUID(as_uuid=True), ForeignKey("techniques.id"), nullable=False)
+    name = Column(String, nullable=False)
+    description = Column(Text, nullable=True)
+    platform = Column(String, nullable=True)
+    procedure_text = Column(Text, nullable=True)
+    tool_used = Column(String, nullable=True)
+    execution_date = Column(DateTime, nullable=True)
+    created_by = Column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=True)
+    result = Column(Enum(TestResult, name="testresult"), nullable=True)
+    state = Column(Enum(TestState, name="teststate"), default=TestState.draft)
+    validated_by = Column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=True)
+    validated_at = Column(DateTime, nullable=True)
+    created_at = Column(DateTime, default=datetime.utcnow)
+
+    # Relationships
+    technique = relationship("Technique", back_populates="tests")
+    evidences = relationship("Evidence", back_populates="test")
+    creator = relationship("User", foreign_keys=[created_by])
+    validator = relationship("User", foreign_keys=[validated_by])