Owner settings: services, hours/buffer, auto-confirm, notify channel (#21)

Adds a session-authenticated /owner/settings blueprint for services CRUD
(create/edit/deactivate), per-resource opening hours + min-notice/max-advance/
buffer, and client-level auto_confirm/notify_channel — all scoped to the
logged-in owner's own client_id. Extends booking_db.py with the missing
tenant-scoped update_service/update_resource/update_client writes, mirroring
the existing update_booking allowlist pattern.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backoffice/app/templates/owner/dashboard.html

@@ -24,7 +24,7 @@
   <div class="card">
     <p>Sie sind angemeldet.</p>
     <p class="muted"><a href="{{ url_for('owner_booking.agenda') }}">Zum Kalender</a></p>
-    <p class="muted">Einstellungen folgen hier.</p>
+    <p class="muted"><a href="{{ url_for('owner_settings.settings') }}">Einstellungen</a></p>
   </div>
   <p class="muted"><a href="{{ url_for('owner_auth.logout') }}">Abmelden</a></p>
 </body>