Trust Caddy's X-Forwarded-Proto so external links use https

url_for(_external=True) (e.g. the owner ICS feed link, #22) was always
guessing "http" since Flask has no way to know the original request was
HTTPS when Caddy proxies to us over plain internal HTTP.

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

backoffice/app/app.py

@@ -12,6 +12,7 @@ from decimal import Decimal
 
 from flask import Flask, jsonify, request, Response
 from waitress import serve
+from werkzeug.middleware.proxy_fix import ProxyFix
 
 import booking_db as bdb
 import db
@@ -24,6 +25,11 @@ from owner_booking import bp as owner_booking_bp
 from owner_settings import bp as owner_settings_bp
 
 app = Flask(__name__, static_folder="static", static_url_path="")
+# Caddy terminates TLS and proxies to us over plain HTTP, forwarding
+# X-Forwarded-Proto/-Host; without this, url_for(_external=True) (e.g. the
+# owner's ICS feed link, #22) always guesses "http" since Flask has no other
+# way to know the original request was HTTPS. One hop of proxy (Caddy).
+app.wsgi_app = ProxyFix(app.wsgi_app, x_proto=1, x_host=1)
 app.register_blueprint(booking_bp)
 app.register_blueprint(public_booking_bp)
 app.register_blueprint(manage_booking_bp)