Add the Home Assistant REST calendar client and the idempotent sync that
writes important school events to Sam's Home Assistant calendar (FR-8). The
school stays read-only; the only write is to the calendar (rule 7).
- internal/calendar: HA REST client (states, create_event, get_events,
delete_event) and the sync that selects important future events, adopts an
existing event by its description marker, records the HA event id on the
item, and records every attempt.
- internal/store: migration 003 (items.ha_event_uid, items.ha_pushed_at and
the append-only calendar_pushes table), candidates, mark-synced and the
push log.
- internal/parse: a school event is important; a routine class period is not.
- cmd/school: `school calendar entities|status|push|events`.
- web: an "Add to calendar" item action and a fail-loud calendar banner, so a
failed write is never swallowed (FR-12).
- scripts/calendar-push.sh, Makefile target, env example, docs and tests.
The target is configurable (SCHOOL_HA_CALENDAR_ENTITY). Home Assistant has no
non-Google writable calendar today, so the entity is asked on AUD-110.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
