Phase 6: Admin portal

Adds is_site_admin-gated site administration: user management (list,
deactivate/reactivate, reset password, promote/demote), room management
(list all rooms including private ones, archive/unarchive, force-transfer
ownership), and an audit log of every admin action.

Backend: User.is_active (deactivation) and Room.is_archived (archive) are
new columns; AdminAuditLog is a new table matching ARCHITECTURE.md's
admin_audit_log design, written to in the same transaction as every
mutating admin action. require_site_admin (dependencies.py) gates all
/api/admin/* routes. get_current_user now rechecks is_active on every
request, so deactivating a user kills their already-open session
immediately, not just future logins. An admin can't deactivate or demote
their own account (the one self-lockout guard included). Archived rooms
drop out of the open-room browse list but stay readable for existing
members.

Frontend: new /admin route (AdminRoute guard, redirects non-admins to
/rooms) with a tabbed Users / Rooms / Audit log / Settings page, plus an
"Admin" link in the account menu for site admins.

Bot/integration management and system settings -- both listed in the
original issue -- are intentionally not here: bot management has nothing
to manage until Phase 7 builds the actual bot data model, and there's no
settings storage or concrete setting to configure yet. Settings has an
empty placeholder tab; bot management is deferred entirely to Phase 7.
Confirmed this scope cut with the repo owner before implementing.

New tests/test_admin.py (14 tests, full suite now 58/58) covers every
admin endpoint's permission gate, the self-action guards, deactivation's
immediate effect on an already-open session, and that every mutating
action produces exactly one audit log row.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -1,8 +1,10 @@
 import { Navigate, Route, Routes } from 'react-router-dom'
 import { AuthProvider } from './context/AuthContext'
+import { AdminRoute } from './components/AdminRoute'
 import { ProtectedRoute } from './components/ProtectedRoute'
 import { LoginPage } from './pages/LoginPage'
 import { ChatShellPage } from './pages/ChatShellPage'
+import { AdminPage } from './pages/AdminPage'
 
 function App() {
   return (
@@ -25,6 +27,14 @@ function App() {
             </ProtectedRoute>
           }
         />
+        <Route
+          path="/admin"
+          element={
+            <AdminRoute>
+              <AdminPage />
+            </AdminRoute>
+          }
+        />
         <Route path="*" element={<Navigate to="/rooms" replace />} />
       </Routes>
     </AuthProvider>