Replace IsAdmin with role-based admin

Switch user admin handling from an AppUser boolean to ASP.NET Identity roles. Removed AppUser.IsAdmin and related configuration/model entries; added migration ReplaceIsAdminWithRoles to copy Users.IsAdmin=true into a persistent admin role and drop the IsAdmin column. CurrentUserResponse now exposes roles (string[]), AuthController returns ordered roles from UserManager, and IdentitySeedService now ensures the admin role exists and assigns/creates an initial admin user in that role. Program.cs registers an Admin-only policy (PolicyNames/RoleNames), adjusts cookie auth events to return 401/403 for API requests, and wires up authorization. Frontend updated to use roles: authSession normalizes roles, adds hasRole and ROLE_ADMIN, router and layout support meta.requiredRoles, and new Forbidden and AdminUsers pages/route are added. codexInfo.md updated to reflect the migration to role-based auth.

API/Security/PolicyNames.cs

@@ -0,0 +1,7 @@
+namespace API.Security
+{
+    public static class PolicyNames
+    {
+        public const string AdminOnly = "admin-only";
+    }
+}

API/Security/RoleNames.cs

@@ -0,0 +1,7 @@
+namespace API.Security
+{
+    public static class RoleNames
+    {
+        public const string Admin = "admin";
+    }
+}