feat: auth-aware UI and role-based access control (#67)

## Summary

- Add `is_admin` column to users table with Alembic migration and a `require_admin` FastAPI dependency that protects all admin-facing write endpoints (games, pokemon, evolutions, bosses, routes CRUD)
- Expose admin status to frontend via user API and update AuthContext to fetch/store `isAdmin` after login
- Make navigation menu auth-aware (different links for logged-out, logged-in, and admin users) and protect frontend routes with `ProtectedRoute` and `AdminRoute` components, preserving deep-linking through redirects
- Fix test reliability: `drop_all` before `create_all` to clear stale PostgreSQL enums from interrupted test runs
- Fix test auth: add `admin_client` fixture and use valid UUID for mock user so tests pass with new admin-protected endpoints

## Test plan

- [x] All 252 backend tests pass
- [ ] Verify non-admin users cannot access admin write endpoints (games, pokemon, evolutions, bosses CRUD)
- [ ] Verify admin users can access admin endpoints normally
- [ ] Verify navigation shows correct links for logged-out, logged-in, and admin states
- [ ] Verify `/admin/*` routes redirect non-admin users with a toast
- [ ] Verify `/runs/new` and `/genlockes/new` redirect unauthenticated users to login, then back after auth

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #67
Co-authored-by: Julian Tabel <juliantabel.jt@gmail.com>
Co-committed-by: Julian Tabel <juliantabel.jt@gmail.com>

frontend/src/components/AdminRoute.tsx

@@ -0,0 +1,35 @@
+import { useEffect, useRef } from 'react'
+import { Navigate, useLocation } from 'react-router-dom'
+import { toast } from 'sonner'
+import { useAuth } from '../contexts/AuthContext'
+
+export function AdminRoute({ children }: { children: React.ReactNode }) {
+  const { user, loading, isAdmin } = useAuth()
+  const location = useLocation()
+  const toastShownRef = useRef(false)
+
+  useEffect(() => {
+    if (!loading && user && !isAdmin && !toastShownRef.current) {
+      toastShownRef.current = true
+      toast.error('Admin access required')
+    }
+  }, [loading, user, isAdmin])
+
+  if (loading) {
+    return (
+      <div className="min-h-screen flex items-center justify-center">
+        <div className="animate-spin rounded-full h-8 w-8 border-b-2 border-accent-500" />
+      </div>
+    )
+  }
+
+  if (!user) {
+    return <Navigate to="/login" state={{ from: location }} replace />
+  }
+
+  if (!isAdmin) {
+    return <Navigate to="/" replace />
+  }
+
+  return <>{children}</>
+}