Use PATCH, fix message, remove username check

Update API/Controllers/Auth/AppUserController.cs: change route attribute to [HttpPatch("{id:guid}")] (fixing verb and missing bracket), correct German error message to indicate the user was deactivated, and remove the redundant existing-username conflict check before SetUserNameAsync. These changes clarify intent and rely on userManager to handle username validation.

API/Controllers/Auth/AppUserController.cs

@@ -36,7 +36,7 @@ namespace API.Controllers.Auth
             return Ok(await user.ToCurrentUserResponseAsync(userManager));
         }
 
-        [HttpPost("{id:guid")]
+        [HttpPatch("{id:guid}")]
         public async Task<IActionResult> UpdateAppUser([FromRoute] Guid id, [FromBody] ChangeUserRequest changeDto)
         {
             var user = await userManager.Users.FirstOrDefaultAsync(x => x.Id == id);
@@ -55,7 +55,7 @@ namespace API.Controllers.Auth
                     var stampResult = await userManager.UpdateSecurityStampAsync(user);
                     if (!stampResult.Succeeded)
                     {
-                        return StatusCode(500, new { message = "Benutzer wurde auf geändert, aber Sessions konnten nicht invalidiert werden. " +
+                        return StatusCode(500, new { message = "Benutzer wurde deaktiviert, aber Sessions konnten nicht invalidiert werden. " +
                                                                "Er könnte also immer noch Angemeldet sein!" });
                     }
                 }
@@ -72,12 +72,6 @@ namespace API.Controllers.Auth
 
                 if (!string.Equals(newUserName, user.UserName, StringComparison.OrdinalIgnoreCase))
                 {
-                    var existing = await userManager.FindByNameAsync(newUserName);
-                    if (existing is not null && existing.Id != user.Id)
-                    {
-                        return Conflict(new { message = "Benutzername ist bereits vergeben." });
-                    }
-
                     var setNameResult = await userManager.SetUserNameAsync(user, newUserName);
                     if (!setNameResult.Succeeded)
                     {