Add OAuth device flow, token introspection, and dynamic client registration

Adds three OAuth surfaces to the OIDC provider so CLIs, terminal agents, and
MCP connectors can authenticate as real users instead of using static API keys.

Device Authorization Grant (RFC 8628):
- OidcDeviceCode model (HMAC device_code, short plaintext user_code, nullable
  user until approval, slow_down polling state), mirroring OidcAuthorizationCode
- POST /oauth/device_authorization issues the code pair + verification URIs
- device_code grant on /oauth/token returns authorization_pending / slow_down /
  access_denied / expired_token, then the standard token triple; single-use
- Authenticated /device approval page, gated by Application#user_allowed?

Token Introspection (RFC 7662):
- POST /oauth/introspect: confidential-caller-authenticated; returns active,
  scope, and the user's groups so resource servers can authorize on membership

Dynamic Client Registration (RFC 7591):
- POST /oauth/register creates public/confidential clients (PKCE required)
- Runtime toggle via a new Setting store + admin switch on the Applications page;
  off by default, env var CLINCH_DCR_ENABLED as bootstrap fallback
- New clients are default-deny (no allowed_groups) until an admin grants access
- RFC 8414 metadata alias at /.well-known/oauth-authorization-server;
  registration_endpoint advertised only while the window is open

Discovery advertises all three grants/endpoints. Seeds add a clinch-cli public
client and a c2a2-introspection confidential client. ADRs in docs/decisions
record the opaque-vs-JWT, device-flow, and DCR-security decisions.

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

@@ -0,0 +1,88 @@
+# User-facing side of the OAuth 2.0 Device Authorization Grant (RFC 8628 §3.3).
+#
+# The CLI/agent sends the human here (GET /device) with the short user_code it
+# was issued. This controller is authenticated, so an unauthenticated visitor is
+# bounced through /signin (with their passkey) and returned here afterwards via
+# session[:return_to_after_authenticating]. On POST /device the signed-in user
+# approves or denies; approval attaches them to the device code and records
+# consent so the token endpoint can mint tokens.
+class DeviceAuthorizationsController < ApplicationController
+  # Browser form endpoint — keep CSRF protection on (do NOT skip it).
+
+  # GET /device?user_code=WDJB-MJHT
+  def show
+    @user_code = params[:user_code].to_s
+    @device_code = OidcDeviceCode.find_by_user_code(@user_code) if @user_code.present?
+
+    if @device_code.nil?
+      @state = @user_code.present? ? :not_found : :prompt
+    elsif @device_code.expired?
+      @state = :expired
+    elsif !@device_code.pending?
+      @state = :already_handled
+    else
+      @state = :confirm
+      @application = @device_code.application
+      @scopes = granted_scopes(@device_code)
+    end
+
+    render :show
+  end
+
+  # POST /device
+  def verify
+    @device_code = OidcDeviceCode.find_by_user_code(params[:user_code].to_s)
+
+    if @device_code.nil?
+      @state = :not_found
+      return render :result
+    end
+
+    if @device_code.expired?
+      @state = :expired
+      return render :result
+    end
+
+    unless @device_code.pending?
+      @state = :already_handled
+      return render :result
+    end
+
+    @application = @device_code.application
+
+    if params[:deny].present?
+      @device_code.deny!
+      @state = :denied
+      return render :result
+    end
+
+    # Enforce the same group-based access control as the OIDC authorize flow.
+    unless @application.user_allowed?(Current.user)
+      @state = :not_allowed
+      return render :result
+    end
+
+    record_consent(@device_code, Current.user)
+    @device_code.approve!(
+      user: Current.user,
+      acr: Current.session.acr,
+      auth_time: Current.session.created_at.to_i
+    )
+    @state = :approved
+    render :result
+  end
+
+  private
+
+  def granted_scopes(device_code)
+    device_code.scope.to_s.split & OidcController::SUPPORTED_SCOPES
+  end
+
+  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 = {}
+    consent.granted_at = Time.current
+    consent.save!
+  end
+end