Phase 6: write important school events to the Home Assistant calendar

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>

scripts/parse-compass.sh

@@ -9,7 +9,7 @@
 #   scripts/parse-compass.sh [--child Harry] [--run latest] [--classify auto]
 #
 # Environment overrides:
-#   IMAGE                image tag                  (default 0.2.0-phase3)
+#   IMAGE                image tag                  (default 0.4.0-phase6)
 #   VOLUME               SQLite volume              (default compass-teams-nhs_compass-teams-nhs-data)
 #   RAW_ROOT             host path to var/raw       (default <repo>/var/raw)
 #   SCHOOL_TIMEZONE      family timezone            (default Australia/Melbourne)
@@ -22,7 +22,7 @@ set -euo pipefail
 
 REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 
-IMAGE="${IMAGE:-compass-teams-nhs/school:0.3.0-phase4}"
+IMAGE="${IMAGE:-compass-teams-nhs/school:0.4.0-phase6}"
 VOLUME="${VOLUME:-compass-teams-nhs_compass-teams-nhs-data}"
 RAW_ROOT="${RAW_ROOT:-$REPO_ROOT/var/raw}"
 SCHOOL_TIMEZONE="${SCHOOL_TIMEZONE:-Australia/Melbourne}"