Document mode:single -> parallel fix on the HA automation

The fix itself lives entirely in HA config (automation.lunch_menu_family_calendar),
not in this repo, so there's nothing to redeploy here -- this just
records what broke and why, for the next time someone (human or Claude)
is debugging why calendar events aren't showing up.

Root cause: mode: single (HA's default) let only one webhook-triggered
run execute at a time; a week push fires 5 POSTs back-to-back, so 4 of 5
got rejected as failed_single and only one day's event was ever created.
Switched to mode: parallel and verified all 5 days land correctly now.

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

CLAUDE.md

@@ -55,7 +55,19 @@ commits), they describe the pre-2026-09-03 app, not the current one.
   `delete_event` — a bad push can't be cleaned up via the HA API, only
   manually in Google Calendar. No duplicate-prevention: re-pushing the
   same date's menu creates a second event — this is why the default
-  schedule is a single day/week, not daily (see below). The webhook id is
+  schedule is a single day/week, not daily (see below).
+  **`mode: parallel`, not the default `single` (fixed 2026-09-03 after a
+  real bug):** a week push fires one webhook POST per school day
+  back-to-back with no delay. Under `mode: single`, HA rejects any
+  webhook trigger that arrives while a previous run is still executing
+  (`calendar.create_event` itself takes ~600-700ms) — 4 of every 5 days
+  were silently dropped as `failed_single`, so only one day's event ever
+  actually got created per week push. Found via `ha_get_automation_traces`
+  after the user reported a manual test push not showing up on the
+  calendar. Each invocation is independent (one `calendar.create_event`
+  call, no shared state), so `parallel` is the correct mode per HA's own
+  guidance ("independent per-entity actions → parallel"), not just a
+  workaround. The webhook id is
   a random token that lives only in the server's `.env` (`HA_WEBHOOK_URL`)
   and in the automation's trigger config — never committed here. If the
   calendar push stops working, check both ends: `GET /api/integrations`