feat(user): Personal access tokens can be regenerated (#38907)

Lets users regenerate a personal access token's value in place, keeping
its name and scopes, instead of deleting and recreating it. Useful when
a token was shared with a third party (e.g. an AI agent) and needs to
be invalidated immediately without redoing scope selection.

Follows the same pattern already used for OAuth2 application client
secrets (`GenerateClientSecret`/`RegenerateSecret`).

**Testing**: added a model unit test and a web integration test;
manually
verified in the running dev server that the old token stops
authenticating
and the new one works immediately after regenerating.

<img width="1040" height="245" alt="image"
src="https://github.com/user-attachments/assets/4de0d8b4-1fc4-49cf-a859-95e24d0b2c0a"
/>

Fixes #38683.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/util/util.go

@@ -6,6 +6,7 @@ package util
 import (
 	"bytes"
 	"crypto/rand"
+	"crypto/subtle"
 	"encoding/hex"
 	"fmt"
 	"math/big"
@@ -99,6 +100,10 @@ func CryptoRandomBytes(length int64) []byte {
 	return buf
 }
 
+func CryptoConstTimeEqual[T string | []byte](a, b T) bool {
+	return subtle.ConstantTimeCompare([]byte(a), []byte(b)) == 1
+}
+
 var chaCha8RandPool = sync.OnceValue(func() *sync.Pool {
 	return &sync.Pool{
 		New: func() any {