Booking confirmation email + customer self-service cancel/reschedule (#18)

Sends a confirmation email (best-effort, fire-and-forget SMTP via mailer.py)
on booking creation, with a manage-booking link embedding the ticket-2 signed
token. Adds /manage/<token>, a stateless cancel/reschedule page that reuses
the existing slot-picker against booking_api's create/cancel/reschedule API,
distinguishing an invalid/expired link from an already-cancelled one. Sender
address uses the client's own domain when configured, falling back to a
mivanchenko.de address otherwise.

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

backoffice/app/app.py

@@ -21,10 +21,12 @@ import db
 from sheets import Sheets
 from booking_api import bp as booking_bp
 from public_booking import bp as public_booking_bp
+from manage_booking import bp as manage_booking_bp
 
 app = Flask(__name__, static_folder="static", static_url_path="")
 app.register_blueprint(booking_bp)
 app.register_blueprint(public_booking_bp)
+app.register_blueprint(manage_booking_bp)
 
 CRM_TOKEN = os.environ.get("CRM_API_TOKEN", "")
 # Separate read-only token for the public iCal feed (calendar apps can't send