Phase 6b/7b: per-child decoded periods, separate brief blocks, idempotent calendar (AUD-119)

- Parse each child's own "Today" day widget for the timetable (FR-16); drop the
  school-wide week calendar's lesson grid and clean the leaked routine rows.
- Add the editable codes map (FR-17): subjects, rooms, teachers and period
  labels, seeded, an unknown code falls back to the raw code; `school codes` CLI.
- Brief: Finn's block first, then Harry's, four sections always present (FR-9);
  periods decoded into plain English; the undated tail is one counted line (FR-18).
- Calendar: write periods + work due + work overdue + major events, per child,
  on every pass. Identity is the stable per-item key, so a re-run never re-inserts
  (FR-8). morning-brief.sh now runs scan -> calendar push -> notify.
- Day-widget labels are 12-hour: 1-6 read as PM, fixing afternoon periods.

Proven live on .13: image 0.6.2-phase6b7b; both children's briefs decoded and
separate; 35 events in calendar.school; runs 2 and 3 of the scan added nothing.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

cmd/school/main.go

@@ -62,6 +62,9 @@ func main() {
 		case "calendar":
 			calendarCommand(os.Args[2:])
 			return
+		case "codes":
+			codesCommand(os.Args[2:])
+			return
 		case "version":
 			fmt.Println(version)
 			return
@@ -97,6 +100,9 @@ func run() error {
 	if err := st.SeedChildren(context.Background(), seedChildren...); err != nil {
 		return err
 	}
+	if err := st.SeedCodes(context.Background()); err != nil {
+		return err
+	}
 
 	am, err := auth.New(cfg.SessionSecret, cfg.SessionTTL, cfg.CookieSecure,
 		cfg.AdminUsername, cfg.AdminPasswordHash, cfg.AdminPassword)