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]>

backend/tests/test_api.py

@@ -125,6 +125,6 @@ def test_tenant_name_is_case_insensitive(client):
 
 
 def test_me_reports_signed_in_user_from_proxy_headers(client):
-    assert client.get("/api/v1/me").json() == {"email": None, "name": None}
+    assert client.get("/api/v1/me").json() == {"email": None, "name": None, "is_admin": False}
     r = client.get("/api/v1/me", headers={"X-Forwarded-Email": "[email protected]", "X-Forwarded-User": "abc-123"})
-    assert r.json() == {"email": "[email protected]", "name": "abc-123"}
+    assert r.json() == {"email": "[email protected]", "name": "abc-123", "is_admin": False}