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/controllers/oidc_pkce_controller_test.rb

@@ -10,6 +10,7 @@ class OidcPkceControllerTest < ActionDispatch::IntegrationTest
       redirect_uris: ["http://localhost:4000/callback"].to_json,
       active: true
     )
+    grant_everyone_access(@application)
 
     # Sign in the user using the test helper
     sign_in_as(@user)