Harden OIDC token endpoints from security review

Fixes from review of the device flow / introspection / DCR work:

Introspection over-disclosure (RFC 7662):
- Restrict introspection to authorized callers — the client a token was issued
  to, or a resource server registered (resource_identifiers) to serve the token's
  bound RFC 8707 audience. Unauthorized callers get {active:false}, disclosing
  nothing, instead of any confidential client reading any token.
- Scope-gate disclosed claims: username only with the email scope, groups only
  with the groups scope (mirrors userinfo). ADR 0005.

Tokens minted for revoked users:
- Re-check application.user_allowed?(user) at mint time in the device, refresh,
  and authorization-code grants (covers app-active, user-active, group
  membership). A user deactivated or removed from the allowed group between
  approval and the token request is refused with access_denied. The refresh
  check runs before rotation so a denied refresh has no side effects.

Device authorization hardening:
- Require confidential clients to authenticate, and require PKCE (code_challenge)
  up front for clients that require it, so an intercepted device_code plus a
  known public client_id cannot redeem tokens.
- Merge (not overwrite) the shared consent record on device approval.

Tests: new oidc_introspection_test and oidc_mint_authorization_test; existing
PKCE/claims tests updated to grant app access (they created ungrouped apps and
relied on the token endpoint not checking authorization). Full suite green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F7cwhwDJp3MJJDoNPVE6zq

app/controllers/device_authorizations_controller.rb

@@ -80,8 +80,12 @@ class DeviceAuthorizationsController < ApplicationController
 
   def record_consent(device_code, user)
     consent = OidcUserConsent.find_or_initialize_by(user: user, application: device_code.application)
-    consent.scopes_granted = granted_scopes(device_code).join(" ")
-    consent.claims_requests = {}
+    # Merge into any existing consent instead of overwriting it. The consent
+    # record is shared with the browser flow (unique on user+application) and
+    # scopes_granted is treated as a granted superset, so a narrower device
+    # request must not shrink previously granted scopes or wipe stored claims.
+    consent.scopes = consent.scopes_granted.to_s.split | granted_scopes(device_code)
+    consent.claims_requests = {} if consent.new_record?
     consent.granted_at = Time.current
     consent.save!
   end