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/schemas/audit.py

@@ -0,0 +1,61 @@
+from datetime import datetime
+from typing import Optional, List, Any
+from pydantic import BaseModel
+
+
+class AuditLogResponse(BaseModel):
+    id: str
+    event_type: str
+    resource_type: str
+    resource_id: Optional[str]
+    user_id: Optional[str]
+    action: str
+    changes: Optional[List[dict]]
+    request_metadata: Optional[dict]
+    sensitivity_level: str
+    checksum: str
+    created_at: datetime
+    user_name: Optional[str] = None
+    user_email: Optional[str] = None
+
+    class Config:
+        from_attributes = True
+
+
+class AuditLogListResponse(BaseModel):
+    logs: List[AuditLogResponse]
+    total: int
+    offset: int
+    limit: int
+
+
+class AuditAlertResponse(BaseModel):
+    id: str
+    audit_log_id: str
+    alert_type: str
+    recipients: List[str]
+    message: Optional[str]
+    is_acknowledged: bool
+    acknowledged_by: Optional[str]
+    acknowledged_at: Optional[datetime]
+    created_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class AuditAlertListResponse(BaseModel):
+    alerts: List[AuditAlertResponse]
+    total: int
+
+
+class IntegrityCheckRequest(BaseModel):
+    start_date: datetime
+    end_date: datetime
+
+
+class IntegrityCheckResponse(BaseModel):
+    total_checked: int
+    valid_count: int
+    invalid_count: int
+    invalid_records: List[str]