feat: implement document management module

- Backend (FastAPI):
  - Attachment and AttachmentVersion models with migration
  - FileStorageService with SHA-256 checksum validation
  - File type validation (whitelist/blacklist)
  - Full CRUD API with version control support
  - Audit trail integration for upload/download/delete
  - Configurable upload directory and file size limit

- Frontend (React + Vite):
  - AttachmentUpload component with drag & drop
  - AttachmentList component with download/delete
  - TaskAttachments combined component
  - Attachments service for API calls

- Testing:
  - 12 tests for storage service and API endpoints

- OpenSpec:
  - add-document-management change archived

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

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

backend/app/schemas/__init__.py

@@ -24,6 +24,10 @@ from app.schemas.audit import (
     AuditLogResponse, AuditLogListResponse, AuditAlertResponse, AuditAlertListResponse,
     IntegrityCheckRequest, IntegrityCheckResponse
 )
+from app.schemas.attachment import (
+    AttachmentResponse, AttachmentListResponse, AttachmentDetailResponse,
+    AttachmentVersionResponse, VersionHistoryResponse
+)
 
 __all__ = [
     "LoginRequest",
@@ -74,4 +78,9 @@ __all__ = [
     "AuditAlertListResponse",
     "IntegrityCheckRequest",
     "IntegrityCheckResponse",
+    "AttachmentResponse",
+    "AttachmentListResponse",
+    "AttachmentDetailResponse",
+    "AttachmentVersionResponse",
+    "VersionHistoryResponse",
 ]