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_test.go

@@ -50,3 +50,31 @@ func TestPeriodLeavesEventsAlone(t *testing.T) {
 		t.Errorf("Period = %q, want the event unchanged", got)
 	}
 }
+
+// TestLabelDecodesEventsAndLessons proves FR-17 covers timetable events too: an
+// ensemble or a concert reads as plain English, its room and teachers decoded,
+// while the event name is kept as the school wrote it. A single-part label is
+// returned unchanged and an unmapped code stays raw.
+func TestLabelDecodesEventsAndLessons(t *testing.T) {
+	codes := testCodes()
+	codes.Room["THEATRE"] = "Theatre"
+	codes.Room["ENS"] = "Ensemble Room"
+	codes.Room["IM3"] = "Room IM3"
+	codes.Teacher["HHL"] = "Mr HHL"
+	codes.Teacher["OFG"] = "Mr OFG"
+	codes.Teacher["KWO"] = "Mr KWO"
+
+	cases := map[string]string{
+		"1 - 7SCF - H14LAB - KNI":                    "Period 1 — Year 7 Science — Room H14LAB — Mr KNI",
+		"JUNIOR GUITAR ENSEMBLE - THEATRE - HHL, OFG": "JUNIOR GUITAR ENSEMBLE — Theatre — Mr HHL / Mr OFG",
+		"JAZZ ENSEMBLE - ENS - KWO":                   "JAZZ ENSEMBLE — Ensemble Room — Mr KWO",
+		"HHL GUITAR E - IM3 - HHL":                    "HHL GUITAR E — Room IM3 — Mr HHL",
+		"Curriculum Day":                              "Curriculum Day",
+		"Concert - HALL":                              "Concert — HALL",
+	}
+	for raw, want := range cases {
+		if got := Label(raw, codes); got != want {
+			t.Errorf("Label(%q) = %q, want %q", raw, got, want)
+		}
+	}
+}