Add an admin page for managing collector API keys

Admins (Microsoft 365 sign-in names in BACKUP_DASH_ADMINS) get an
Admin link to /admin.html, where they can create, list and revoke the
API keys collectors use, instead of running manage.py on the server.

Backend
- /api/v1/admin/keys (list, create, revoke), admins only (403
  otherwise); the user comes from the headers nginx sets after sign-in
- Changes need an X-Requested-By header, which another site's page
  can't add, so a signed-in admin's browser can't be used to forge them
- A new key is returned once; only its hash is stored. Names are unique
  for good, so revoked keys' history stays unambiguous
- Keys record who created/revoked them and the company they last
  reported for (schema v2; databases migrate automatically on start)
- /api/v1/me reports is_admin; manage.py shares the key code and
  records itself as creator/revoker

Frontend
- admin.html / admin.js: name validation shown after interaction
  (:user-invalid, aria-invalid kept in sync), the one-time key with a
  copy button (falls back to selecting it if clipboard access is
  refused), a key table flagging keys quiet for over 24 hours, and
  revoking confirmed in a <dialog>; results announced in a status region
- Shared el() and signed-in helpers moved to dom.js / signed-in.js
- Mock mode (?mock=1) includes sample keys

Tested: 15 new backend tests (access, forged requests, names,
duplicates, revoking, last-used company, v1 -> v2 upgrade); in the
browser, every admin flow in mock mode and at phone width; end to end
through the nginx config with the real API (a key created on the API
works for ingest; revoking it makes ingest 401; non-admins see an
explanation and no Admin link).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

public/index.html

@@ -14,6 +14,7 @@
     <div class="topbar__meta">
       <span id="last-updated" class="muted" aria-live="polite">Loading…</span>
       <button id="refresh-btn" class="btn" type="button">Refresh</button>
+      <a id="admin-link" class="btn" href="/admin.html" hidden>Admin</a>
       <!-- Filled in from /api/v1/me when signed in with Microsoft 365 -->
       <span id="signed-in" class="topbar__user" hidden>
         <span id="signed-in-email" class="muted"></span>