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/owner_auth.py

@@ -53,9 +53,8 @@ def logout():
 @bp.get("/")
 @login_required
 def dashboard():
-    # Tickets 6/7/8 (calendar, manual booking, settings) build the real
-    # dashboard content on top of this session; this is just the landing
-    # page proving a login resolved to (and is scoped to) one client_id.
+    # Tickets 7/8 (settings, notify channel) still build on top of this
+    # session; the agenda/calendar (#20) now lives at owner_booking.agenda.
     client = bdb.get_client(session["client_id"])
     return render_template("owner/dashboard.html", client=client)