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/user.rb

@@ -42,7 +42,18 @@ class User < ApplicationRecord
   enum :status, {active: 0, disabled: 1, pending_invitation: 2}
 
   # Scopes
-  scope :admins, -> { where(admin: true) }
+  scope :admins, -> { joins(:groups).where(groups: {admin: true}).distinct }
+
+  # Set true on a user (or on the user_params) to skip the auto-assign callback
+  # for that record. Used by the admin invite form (opt-out checkbox) and by
+  # tests that want a clean slate.
+  attr_accessor :skip_auto_assign
+
+  after_create :add_to_auto_assign_groups, unless: :skip_auto_assign
+
+  def admin?
+    groups.any?(&:admin?)
+  end
 
   # TOTP methods
   def totp_enabled?
@@ -222,6 +233,10 @@ class User < ApplicationRecord
 
   private
 
+  def add_to_auto_assign_groups
+    Group.auto_assign.each { |g| groups << g }
+  end
+
   def no_reserved_claim_names
     return if custom_claims.blank?