Phase 5: per-child dashboard (overdue first), item detail, delete

- internal/dashboard: classify items into overdue / due today / due this week /
  no due date / messages; render the plain-text brief (FR-6, FR-7, FR-9).
- internal/store: ItemsForChild, ItemByID, DeleteItemByID, ScanSummaryForChild
  and the stored-brief CRUD helpers (FR-10).
- internal/web: routes /?child=, /items/{id}, POST /items/{id}/delete,
  POST /summaries/build, POST /summaries/{id}/delete, plus a per-child scan
  banner (FR-12) and a phone-first server-rendered template.
- Calendar events are never "overdue": a past lesson is history, not work.
- cmd/school brief build|list; scripts/screenshot-phase5.mjs for phone evidence.
- Tests: bucket classification, stored-brief round-trip, and HTTP routes
  including the NO SCAN / SCAN FAILED / healthy banner behaviour.

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

cmd/school/main.go

@@ -53,6 +53,9 @@ func main() {
 		case "items":
 			itemsCommand(os.Args[2:])
 			return
+		case "brief":
+			briefCommand(os.Args[2:])
+			return
 		case "version":
 			fmt.Println(version)
 			return