feat: implement audit trail module

- Backend (FastAPI):
  - AuditLog and AuditAlert models with Alembic migration
  - AuditService with SHA-256 checksum for log integrity
  - AuditMiddleware for request metadata extraction (IP, user_agent)
  - Integrated audit logging into Task, Project, Blocker APIs
  - Query API with filtering, pagination, CSV export
  - Integrity verification endpoint
  - Sensitive operation alerts with acknowledgement

- Frontend (React + Vite):
  - Admin AuditPage with filters and export
  - ResourceHistory component for change tracking
  - Audit service for API calls

- Testing:
  - 15 tests covering service and API endpoints

- OpenSpec:
  - add-audit-trail change archived

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -10,8 +10,11 @@ from app.models.comment import Comment
 from app.models.mention import Mention
 from app.models.notification import Notification
 from app.models.blocker import Blocker
+from app.models.audit_log import AuditLog, AuditAction, SensitivityLevel, EVENT_SENSITIVITY, ALERT_EVENTS
+from app.models.audit_alert import AuditAlert
 
 __all__ = [
     "User", "Role", "Department", "Space", "Project", "TaskStatus", "Task", "WorkloadSnapshot",
-    "Comment", "Mention", "Notification", "Blocker"
+    "Comment", "Mention", "Notification", "Blocker",
+    "AuditLog", "AuditAlert", "AuditAction", "SensitivityLevel", "EVENT_SENSITIVITY", "ALERT_EVENTS"
 ]