Owner calendar view + manual booking + owner cancel/reschedule (#20)

Adds an owner-authenticated weekly agenda (grouped by day, today highlighted)
with manual walk-in/phone booking creation, cancel, and reschedule -- all
routed through booking_api.py's create/cancel/reschedule logic (refactored
into shared helpers) so the EXCLUDE overlap constraint and confirmation
email stay on the single existing code path. Manual creation can skip the
opening-hours/min-notice/max-advance/buffer checks via an explicit override,
but never the overlap constraint itself.

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

backoffice/app/templates/owner/dashboard.html

@@ -23,7 +23,8 @@
   <h1>{{ client.business_name if client else 'Mein Konto' }}</h1>
   <div class="card">
     <p>Sie sind angemeldet.</p>
-    <p class="muted">Kalender, Buchungen und Einstellungen folgen hier.</p>
+    <p class="muted"><a href="{{ url_for('owner_booking.agenda') }}">Zum Kalender</a></p>
+    <p class="muted">Einstellungen folgen hier.</p>
   </div>
   <p class="muted"><a href="{{ url_for('owner_auth.logout') }}">Abmelden</a></p>
 </body>