Fix code-review findings: ajax error handling, translated comment, docs

- js/index.js: the occ-command ajax call used the wrong jQuery .fail()
  signature ((response, code) instead of (jqXHR, textStatus,
  errorThrown)), so any failure printed the literal string
  "[object Object]" instead of a useful message.
- js/index.js: the initial bootstrap call that builds the terminal had
  no .fail() handler at all - a failure there (expired session/CSRF
  token, network error, rejected admin check) left #app-content
  permanently blank with zero diagnostic information. This is exactly
  what made today's NC34 debugging session harder than it needed to
  be; now it shows the actual HTTP status/reason in the page.
- OccController.php: translated a leftover Dutch comment (missed by
  the earlier Russian->English pass since it wasn't Russian), and
  documented why requireAdmin() is intentional defense-in-depth rather
  than a misunderstanding of the framework: verified against
  Nextcloud's own SecurityMiddleware source that any controller method
  without @NoAdminRequired already rejects non-admins by default, so
  this check (and the original "security fix" commit that added it)
  was addressing a non-issue for THIS controller specifically.
  DbController's equivalent check is still genuinely load-bearing -
  that controller originally shipped with @PublicPage, which does
  bypass the default protection - so it gets a note explaining the
  distinction instead.
- README: added compatibility matrix rows for 0.4.0 (app id rename to
  extended_occweb) and 0.4.1 (these fixes), both verified on NC 34.0.4.

Deferred to when a test instance exists (tracked, not fixed here): no
documented migration path from the old occweb id to extended_occweb
for existing installs, OccController's constructor doing expensive
work before the (redundant) admin check runs, and js/jquery.js loading
unconditionally on NC30-33 where core still provides jQuery.

Version bump to 0.4.1.

lib/Controller/DbController.php

@@ -162,7 +162,12 @@ class DbController extends Controller
 
     public function query()
     {
-        // Check admin privileges
+        // Deliberate defense-in-depth (see the equivalent check in
+        // OccController::requireAdmin() for why it's kept even though
+        // Nextcloud's SecurityMiddleware already enforces this by default
+        // absent @NoAdminRequired) - this one is also the direct fix for
+        // this controller having shipped with @PublicPage originally,
+        // which did bypass that default protection entirely.
         $user = $this->userSession->getUser();
         if (!$user) {
             return new JSONResponse(['error' => 'Not authenticated'], 401);