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/mailer.py

@@ -0,0 +1,59 @@
+"""Best-effort SMTP email sending (#18).
+
+Mirrors app.py's DB -> Sheets mirror pattern: a single background worker
+thread drains a queue, and a send failure is logged, never raised back to the
+caller -- booking creation must succeed even if the mail relay is down.
+"""
+import os
+import queue
+import smtplib
+import threading
+import traceback
+from email.message import EmailMessage
+
+SMTP_HOST = os.environ.get("SMTP_HOST", "")
+SMTP_PORT = int(os.environ.get("SMTP_PORT", "587"))
+SMTP_USERNAME = os.environ.get("SMTP_USERNAME", "")
+SMTP_PASSWORD = os.environ.get("SMTP_PASSWORD", "")
+# Used when a client has no domain configured -- see booking_mail.py.
+MAIL_FALLBACK_FROM = os.environ.get("MAIL_FALLBACK_FROM", "noreply@mivanchenko.de")
+
+_queue = queue.Queue()
+
+
+def _send_now(msg):
+    if not SMTP_HOST:
+        print(f"[mailer] SMTP_HOST not configured, skipping send to {msg['To']}", flush=True)
+        return
+    with smtplib.SMTP(SMTP_HOST, SMTP_PORT, timeout=10) as smtp:
+        smtp.starttls()
+        if SMTP_USERNAME:
+            smtp.login(SMTP_USERNAME, SMTP_PASSWORD)
+        smtp.send_message(msg)
+
+
+def _worker():
+    while True:
+        msg = _queue.get()
+        try:
+            _send_now(msg)
+        except Exception:  # noqa: BLE001
+            print(f"[mailer] send to {msg['To']} failed:", flush=True)
+            traceback.print_exc()
+        finally:
+            _queue.task_done()
+
+
+threading.Thread(target=_worker, daemon=True).start()
+
+
+def send_email(to_addr, subject, html_body, from_addr=None):
+    """Queue an HTML email for best-effort async delivery. Never raises and
+    never blocks the caller on the network -- the actual SMTP conversation
+    happens on the background worker thread."""
+    msg = EmailMessage()
+    msg["Subject"] = subject
+    msg["From"] = from_addr or MAIL_FALLBACK_FROM
+    msg["To"] = to_addr
+    msg.set_content(html_body, subtype="html")
+    _queue.put(msg)