Phase 6b/7b: dashboard codes editor, decode events, human-readable briefs (AUD-115)

- New /codes dashboard page (behind the session login, linked in the top bar):
  lists every mapping by kind with Add, inline Edit+Save and Delete. Sam can put
  real teacher surnames in without a rebuild (FR-17).
- Decode timetable EVENTS through the same map as periods, so an ensemble reads
  'JAZZ ENSEMBLE - Ensemble Room - Mr KWO' instead of raw codes.
- teacherName splits comma- or space-separated codes ('HHL, OFG').
- SeedCodes is a one-time bootstrap while the table is empty, so a correction or
  a delete is never overwritten or restored.
- Tests: decode Label/Event, the /codes page (save, refuse, csrf, delete, auth),
  store re-seed behaviour. go test ./internal/... passes.

internal/dashboard/dashboard.go

@@ -419,7 +419,7 @@ func RenderBrief(child string, b Buckets, decode func(string) string, loc *time.
 	fmt.Fprintf(&sb, "=== %s ===\n", strings.ToUpper(strings.TrimSpace(child)))
 
 	writeBriefPeriods(&sb, b.PeriodsToday, decode, loc)
-	writeBriefEvents(&sb, b.EventsToday, loc)
+	writeBriefEvents(&sb, b.EventsToday, decode, loc)
 	writeBriefWork(&sb, "OVERDUE", b.Overdue, loc)
 	writeBriefFuture(&sb, b, loc)
 	return sb.String()
@@ -455,19 +455,25 @@ func writeBriefPeriods(sb *strings.Builder, items []store.Item, decode func(stri
 	}
 }
 
-// writeBriefEvents lists today's major school events (FR-9b).
-func writeBriefEvents(sb *strings.Builder, items []store.Item, loc *time.Location) {
+// writeBriefEvents lists today's major school events (FR-9b), decoded through
+// the same map as periods so an ensemble or a concert reads as plain English
+// (FR-17). Unknown codes stay raw.
+func writeBriefEvents(sb *strings.Builder, items []store.Item, decode func(string) string, loc *time.Location) {
 	sb.WriteString("\nEVENTS TODAY\n")
 	if len(items) == 0 {
 		sb.WriteString("- none\n")
 		return
 	}
 	for _, it := range items {
+		title := strings.TrimSpace(it.Title)
+		if decode != nil {
+			title = decode(title)
+		}
 		when := ""
 		if it.DueAt != nil {
 			when = " (" + it.DueAt.In(loc).Format("15:04") + ")"
 		}
-		fmt.Fprintf(sb, "- %s%s\n", strings.TrimSpace(it.Title), when)
+		fmt.Fprintf(sb, "- %s%s\n", title, when)
 	}
 }