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/decode/decode.go

@@ -38,11 +38,12 @@ func Period(title string, m store.CodeMap) string {
 }
 
 // teacherName decodes a teacher field. Compass sometimes carries several codes
-// in the one field ("GT SMH"), so every token is decoded and joined.
+// in the one field, separated by spaces ("GT SMH") or commas ("HHL, OFG"), so
+// every token is decoded and joined. An unmapped token stays raw (FR-17).
 func teacherName(m store.CodeMap, field string) string {
-	tokens := strings.Fields(field)
-	if len(tokens) <= 1 {
-		return codeName(m, store.CodeTeacher, field)
+	tokens := teacherTokens(field)
+	if len(tokens) == 0 {
+		return strings.TrimSpace(field)
 	}
 	out := make([]string, 0, len(tokens))
 	for _, tok := range tokens {
@@ -51,6 +52,43 @@ func teacherName(m store.CodeMap, field string) string {
 	return strings.Join(out, " / ")
 }
 
+// teacherTokens splits a teacher field on commas and whitespace.
+func teacherTokens(field string) []string {
+	return strings.Fields(strings.ReplaceAll(field, ",", " "))
+}
+
+// Label renders any Compass timetable label as plain English: a lesson label
+// through Period, anything else (an ensemble, a concert, an excursion) through
+// Event. It is the decoder the briefs use. An unmapped code stays raw (FR-17).
+func Label(title string, m store.CodeMap) string {
+	if _, _, _, _, ok := period.Split(strings.TrimSpace(title)); ok {
+		return Period(title, m)
+	}
+	return Event(title, m)
+}
+
+// Event renders a timetable event label as plain English:
+//
+//	"JUNIOR GUITAR ENSEMBLE - THEATRE - HHL, OFG" -> "JUNIOR GUITAR ENSEMBLE — Theatre — Mr HHL / Mr OFG"
+//
+// The event name is kept as the school wrote it; the room and teacher codes are
+// decoded. A label with no room or teacher is returned with just its name, and
+// an unmapped code stays raw (FR-17).
+func Event(title string, m store.CodeMap) string {
+	parts := period.SplitDash(strings.TrimSpace(title))
+	if len(parts) <= 1 {
+		return strings.TrimSpace(title)
+	}
+	out := []string{parts[0]}
+	if parts[1] != "" {
+		out = append(out, codeName(m, store.CodeRoom, parts[1]))
+	}
+	if len(parts) >= 3 && parts[2] != "" {
+		out = append(out, teacherName(m, parts[2]))
+	}
+	return strings.Join(out, " — ")
+}
+
 // periodName renders the period slot: a word label such as "C" uses the map
 // ("Connect"), a number becomes "Period 3", and anything unknown stays raw.
 func periodName(slot string, m store.CodeMap) string {