feat(notifications): add Telegram test button

New "Test senden" button next to Speichern posts current form
credentials (not DB) to /actions/notifications/test, which fires a
test message and redirects back with a flash chip showing the outcome
(including the Telegram API's error description on failure).

telegram_send is now public and returns (ok, detail) so the UI can
surface real error messages ("chat not found", "Unauthorized", etc.).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/templates/_settings_notifications.html

@@ -3,6 +3,18 @@
     Wähle einen Kanal und entscheide, welche Events dich erreichen sollen.
 </p>
 
+{% if notif_flash %}
+<div class="chip mb-4
+    {% if notif_flash == 'test_ok' %}chip-ok
+    {% else %}chip-bad
+    {% endif %}">
+    {% if notif_flash == 'test_ok' %}Testnachricht gesendet.
+    {% elif notif_flash == 'test_missing' %}Bot-Token und Chat-ID eintragen, um zu testen.
+    {% elif notif_flash == 'test_fail' %}Test fehlgeschlagen{% if notif_flash_detail %}: {{ notif_flash_detail }}{% endif %}.
+    {% endif %}
+</div>
+{% endif %}
+
 <form method="post" action="/actions/notifications" class="space-y-4 max-w-xl"
       autocomplete="off" data-lpignore="true" data-1p-ignore data-bwignore data-form-type="other">
     <input type="hidden" name="csrf" value="{{ csrf }}">
@@ -42,5 +54,9 @@
         </label>
     </div>
 
-    <button class="btn btn-primary" type="submit">Speichern</button>
+    <div class="flex gap-2">
+        <button class="btn btn-primary" type="submit">Speichern</button>
+        <button class="btn btn-ghost" type="submit"
+                formaction="/actions/notifications/test" formnovalidate>Test senden</button>
+    </div>
 </form>