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/app/auth.py

@@ -1,14 +1,16 @@
-"""Collector (machine) authentication.
+"""Authentication and authorisation.
 
-Dashboard *users* are authenticated by nginx in front of the app (Microsoft 365 sign-in via
-oauth2-proxy), so nothing here deals with people.
+Collectors (machines) authenticate with API keys, checked here. Dashboard *users* sign in with
+Microsoft 365 in nginx (via oauth2-proxy), which then tells the app who they are in
+X-Forwarded-* headers it sets itself (a browser can't supply them). The app only decides whether
+that user is an admin.
 """
 
 import hashlib
 import secrets
 import sqlite3
 
-from fastapi import Depends, Header, HTTPException
+from fastapi import Depends, Header, HTTPException, Request
 
 from .db import get_db
 from .timeutil import to_db, utcnow
@@ -25,6 +27,35 @@ def hash_key(key: str) -> str:
     return hashlib.sha256(key.encode()).hexdigest()
 
 
+def current_user(request: Request) -> str | None:
+    """The signed-in user's sign-in name, lowercase; None without Microsoft sign-in (e.g. dev)."""
+    user = request.headers.get("x-forwarded-email") or request.headers.get("x-forwarded-preferred-username")
+    return user.strip().lower() if user and user.strip() else None
+
+
+def is_admin(request: Request) -> bool:
+    user = current_user(request)
+    return user is not None and user in request.app.state.settings.admins
+
+
+def require_admin(request: Request) -> str:
+    if not is_admin(request):
+        raise HTTPException(403, "Admins only. Admins are listed in BACKUP_DASH_ADMINS on the server.")
+    return current_user(request)
+
+
+# Changes made from the admin page must carry this header. A page on another site can't add a
+# custom header to a request to the dashboard (the browser would first ask the API, which allows
+# no other origins), so a signed-in admin's browser can't be used to forge them.
+PAGE_HEADER = "X-Requested-By"
+PAGE_HEADER_VALUE = "backup-dashboard"
+
+
+def require_page_request(x_requested_by: str | None = Header(None)) -> None:
+    if x_requested_by != PAGE_HEADER_VALUE:
+        raise HTTPException(403, f"Missing {PAGE_HEADER} header")
+
+
 def require_ingest_key(
     authorization: str | None = Header(None),
     conn: sqlite3.Connection = Depends(get_db),