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/manage_booking.py

@@ -0,0 +1,51 @@
+"""Customer self-service manage-booking page (#18), reached via the
+token-linked link sent in the confirmation email (booking_mail.py). The
+signed token (booking_api.verify_manage_token) is the only source of
+identity here -- customers have no account, so this route needs no login.
+
+The actual cancel/reschedule mutations still go through booking_api.py's
+JSON API (#16); this route only resolves the token, decides which of the
+three states (invalid/expired, already used, active) to render, and lets the
+page's own JS drive the API from there.
+"""
+from flask import Blueprint, render_template
+from zoneinfo import ZoneInfo
+
+import booking_db as bdb
+from booking_api import verify_manage_token
+
+bp = Blueprint("manage_booking", __name__)
+
+
+@bp.get("/manage/<token>")
+def manage_page(token):
+    resolved = verify_manage_token(token)
+    if resolved is None:
+        # Distinct from "already used" per #18's acceptance criteria -- an
+        # expired/malformed token never resolved to a booking at all.
+        return render_template("manage.html", state="invalid"), 400
+
+    client_id, booking_id = resolved
+    booking = bdb.get_booking(client_id, booking_id)
+    client = bdb.get_client(client_id)
+    if booking is None or client is None:
+        return render_template("manage.html", state="invalid"), 404
+
+    if booking["status"] == "cancelled":
+        return render_template("manage.html", state="used")
+
+    tz = ZoneInfo(client.get("timezone") or "Europe/Berlin")
+    duration_minutes = int(
+        (booking["end_time"] - booking["start_time"]).total_seconds() // 60)
+    return render_template(
+        "manage.html",
+        state="active",
+        token=token,
+        client_id=client_id,
+        booking_id=booking_id,
+        resource_id=booking["resource_id"],
+        service=booking["service"],
+        duration_minutes=duration_minutes,
+        start_local=booking["start_time"].astimezone(tz),
+        status=booking["status"],
+    )