feat: add UUID utility functions for attachment system

- Add NewUUID() function for generating unique attachment identifiers
- Add IsValidUUID() function for validating UUID format in API requests
- Support 32-character hex string validation for secure file handling

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

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

core/util/util.go

@@ -3,6 +3,7 @@ package util
 import (
 	"crypto/rand"
 	"encoding/hex"
+	"regexp"
 	"time"
 )
 
@@ -44,3 +45,23 @@ func NewInviteId() (string, error) {
 
 	return hex.EncodeToString(byteArray), nil
 }
+
+func NewUUID() string {
+	guid, err := NewGuid()
+	if err != nil {
+		// Fallback to timestamp-based UUID if random generation fails
+		return hex.EncodeToString([]byte(time.Now().Format("20060102150405")))
+	}
+	return guid
+}
+
+func IsValidUUID(uuid string) bool {
+	// Check if the string is a valid 32-character hex string (16 bytes * 2 hex chars)
+	if len(uuid) != 32 {
+		return false
+	}
+	
+	// Check if all characters are valid hex characters
+	matched, _ := regexp.MatchString("^[0-9a-f]{32}$", uuid)
+	return matched
+}