feat: secure admin panel with Supabase auth + course management CRUD

- Replace ADMIN_SECRET query param with proper Supabase auth + is_admin flag
- Add admin layout with auth check (redirects non-admin to /)
- Add AdminShell component with sidebar navigation (Dashboard, Candidatures, Cours)
- Add admin dashboard with stats (candidatures, users, modules)
- Add admin candidatures page with filters and approve/reject
- Add admin course management page (create, edit, delete, publish/unpublish)
- Add API routes: GET/POST /api/admin/modules, GET/PUT/DELETE /api/admin/modules/[id]
- Add verifyAdmin() helper for API route protection
- Update database types with is_admin on profiles

https://claude.ai/code/session_01H2aRGDaKgarPvhay2HxN6Y

lib/admin.ts

@@ -0,0 +1,40 @@
+import { createClient, createAdminClient } from "@/lib/supabase/server";
+import type { Profile } from "@/types/database.types";
+
+// Vérifie que l'utilisateur connecté est admin
+// Utilisé dans les API routes admin
+export async function verifyAdmin(): Promise<{ admin: Profile } | { error: string; status: number }> {
+  const supabase = await createClient();
+
+  const {
+    data: { user },
+    error: authError,
+  } = await supabase.auth.getUser();
+
+  if (authError || !user) {
+    return { error: "Non authentifié.", status: 401 };
+  }
+
+  // Utiliser le client admin pour lire le profil (pas de RLS)
+  const adminClient = createAdminClient();
+  const { data: profile, error: profileError } = await adminClient
+    .from("profiles")
+    .select("*")
+    .eq("id", user.id)
+    .single();
+
+  if (profileError || !profile) {
+    return { error: "Profil introuvable.", status: 404 };
+  }
+
+  if (!(profile as Profile).is_admin) {
+    return { error: "Accès refusé.", status: 403 };
+  }
+
+  return { admin: profile as Profile };
+}
+
+// Helper pour répondre avec erreur si non-admin
+export function isAdminError(result: { admin: Profile } | { error: string; status: number }): result is { error: string; status: number } {
+  return "error" in result;
+}