Default-deny access control with group flags and access enumeration

Replaces the implicit "empty allowed_groups means public" rule with
explicit default-deny across both OIDC and ForwardAuth. Adds two boolean
flags on Group — auto_assign (Keycloak-style auto-join on user create)
and admin (members can reach the admin panel) — and drops the
users.admin column entirely. Adds "Users with access" and "Accessible
applications" panels with via-group badges on the application/user show
pages.

BEHAVIOR CHANGE: a ForwardAuth app with no allowed_groups previously
bypassed authentication entirely; it now returns 403 like any other
unauthorized request. The data migration seeds an "everyone" group and
attaches it to all previously group-less apps to preserve behavior on
existing installs. An "admins" group is seeded and backfilled from any
user with the old admin column.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/services/oidc_jwt_service_test.rb

@@ -95,7 +95,8 @@ class OidcJwtServiceTest < ActiveSupport::TestCase
   end
 
   test "admin claim should not be included in token" do
-    @user.update!(admin: true)
+    # alice is already in admin_group via fixtures, so admin? is true here
+    assert @user.admin?
 
     token = @service.generate_id_token(@user, @application)