feat: implement secure file upload system with JWT authentication

- Add JWT-based secure file access for local storage with 1-hour expiry
- Implement GORM repository methods for attachment CRUD operations
- Add secure file serving endpoint with token validation
- Update storage interface to support user context in URL generation
- Add comprehensive security features including path traversal protection
- Update documentation with security model and configuration examples
- Add utility functions for hex/byte conversion and UUID validation
- Configure secure file permissions (0600) for uploaded files

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

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

core/api/attachment_storage.go

@@ -162,8 +162,8 @@ func GetAttachmentDownloadURL(w rest.ResponseWriter, r *rest.Request) {
 		return
 	}
 
-	// Generate download URL (valid for 1 hour)
-	url, err := attachmentHandler.storage.GetURL(attachment.FilePath, time.Hour)
+	// Generate download URL (valid for 1 hour) with user context for JWT tokens
+	url, err := attachmentHandler.storage.GetURLWithContext(attachment.FilePath, time.Hour, user.Id, attachment.OrgId)
 	if err != nil {
 		rest.Error(w, "Failed to generate download URL", http.StatusInternalServerError)
 		return