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/util/util.go

@@ -65,3 +65,13 @@ func IsValidUUID(uuid string) bool {
 	matched, _ := regexp.MatchString("^[0-9a-f]{32}$", uuid)
 	return matched
 }
+
+// HexToBytes converts a hex string to bytes
+func HexToBytes(hexString string) ([]byte, error) {
+	return hex.DecodeString(hexString)
+}
+
+// BytesToHex converts bytes to a hex string
+func BytesToHex(bytes []byte) string {
+	return hex.EncodeToString(bytes)
+}