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>

deploy/booking/RATE_LIMIT.md

@@ -1,11 +1,11 @@
-# Caddy rate limiting for `/book/*` and `/api/booking/*` (#17)
+# Caddy rate limiting for `/book/*`, `/manage/*` and `/api/booking/*` (#17, #18)
 
 Like every other homelab Caddy change (see `deploy/clients/new-client.sh`), there's no
 Caddyfile tracked in this repo -- apply this by hand at `/etc/caddy/Caddyfile` on the host
 and reload with `docker exec caddy caddy reload --config /etc/caddy/Caddyfile`.
 
 Add, on the site block that proxies to `smb-crm` (e.g. `onboard.mivanchenko.de`, or wherever
-`/book/` and `/api/booking/` are routed):
+`/book/`, `/manage/` and `/api/booking/` are routed):
 
 ```
 handle /book/* {
@@ -18,6 +18,16 @@ handle /book/* {
     }
     reverse_proxy smb-crm:8080
 }
+handle /manage/* {
+    rate_limit {
+        zone book_public {
+            key {remote_host}
+            events 20
+            window 1m
+        }
+    }
+    reverse_proxy smb-crm:8080
+}
 handle /api/booking/* {
     rate_limit {
         zone book_api {