fix: use url-unsafe base 64 for auth tokens

.net throws an error when decoding url-safe base 64
luckily we never decode it *except* for tokens, so those can keep using
url-unsafe base 64. they're never used in URLs after all

Foxnouns.Backend/Services/Auth/AuthService.cs

@@ -358,7 +358,7 @@ public class AuthService(
 
     private static (string, byte[]) GenerateToken()
     {
-        string token = AuthUtils.RandomToken();
+        string token = AuthUtils.RandomUrlUnsafeToken();
         byte[] hash = SHA512.HashData(Convert.FromBase64String(token));
 
         return (token, hash);