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>

app/models/group.rb

@@ -15,6 +15,11 @@ class Group < ApplicationRecord
   normalizes :name, with: ->(name) { name.strip.downcase }
   validate :no_reserved_claim_names
 
+  scope :auto_assign, -> { where(auto_assign: true) }
+  scope :admin, -> { where(admin: true) }
+
+  before_destroy :ensure_other_admin_group_exists
+
   # Parse custom_claims JSON field
   def parsed_custom_claims
     return {} if custom_claims.blank?
@@ -23,6 +28,13 @@ class Group < ApplicationRecord
 
   private
 
+  def ensure_other_admin_group_exists
+    return unless admin?
+    return if Group.where(admin: true).where.not(id: id).exists?
+    errors.add(:base, "cannot delete the last administrators group")
+    throw :abort
+  end
+
   def no_reserved_claim_names
     return if custom_claims.blank?