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/web/templates/dashboard.html

@@ -17,6 +17,7 @@
       Compass / Teams
     </a>
     <div class="nav-bar__right">
+      <a class="button-utility" href="/codes">Codes</a>
       <button class="button-utility" type="button" data-theme-toggle>Dark</button>
       <span class="nav-bar__user">{{.Username}}</span>
       <form method="post" action="/logout" class="nav-bar__logout">