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/main.py

@@ -14,6 +14,7 @@ from app.api.notifications import router as notifications_router
 from app.api.blockers import router as blockers_router
 from app.api.websocket import router as websocket_router
 from app.api.audit import router as audit_router
+from app.api.attachments import router as attachments_router
 from app.core.config import settings
 
 app = FastAPI(
@@ -47,6 +48,7 @@ app.include_router(notifications_router)
 app.include_router(blockers_router)
 app.include_router(websocket_router)
 app.include_router(audit_router)
+app.include_router(attachments_router)
 
 
 @app.get("/health")