Add Home Assistant calendar sync via webhook

Every successful menu fetch now POSTs the day's items to HA_WEBHOOK_URL
(when set), so an HA automation can turn it into a calendar event. Surface
the push result as a "Calendar Sync" badge on the status card, hidden
unless HA_WEBHOOK_URL is configured.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KsJqAZ82Veyf5oihBr71S5

CLAUDE.md

@@ -20,6 +20,12 @@ Runs as one Docker container on the user's Unraid home server.
   first time, before a settings file exists.
 - Vacation ranges suppress sends without disabling the schedule — checked
   fresh on every run via `is_on_vacation()`.
+- **Home Assistant integration (2026-09-02):** if `HA_WEBHOOK_URL` is set,
+  every successful menu fetch also POSTs `{"date": ..., "items": [...]}` to
+  that webhook, independent of whether the SMS send itself succeeds. An HA
+  automation (webhook trigger → `calendar.create_event`) turns this into an
+  all-day calendar event with the menu. This makes the scrape here the
+  single source of truth — HA doesn't poll or re-scrape anything.
 
 ## Local development