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/views/admin/applications/index.html.erb

@@ -23,6 +23,30 @@
   </div>
 </dl>
 
+<% dcr_on = Application.dynamic_registration_enabled? %>
+<div class="mt-4 rounded-lg border px-4 py-3 flex items-center justify-between <%= dcr_on ? "border-amber-300 bg-amber-50 dark:border-amber-700 dark:bg-amber-900/20" : "border-gray-200 bg-white dark:border-gray-700 dark:bg-gray-800" %>">
+  <div class="pr-4">
+    <p class="text-sm font-medium text-gray-900 dark:text-gray-100">
+      Dynamic client registration
+      <span class="ml-2 inline-flex items-center rounded-full px-2 py-0.5 text-xs font-semibold <%= dcr_on ? "bg-amber-200 text-amber-900 dark:bg-amber-800 dark:text-amber-100" : "bg-gray-200 text-gray-700 dark:bg-gray-700 dark:text-gray-200" %>">
+        <%= dcr_on ? "On" : "Off" %>
+      </span>
+    </p>
+    <p class="mt-1 text-xs text-gray-600 dark:text-gray-400">
+      <% if dcr_on %>
+        Clients can self-register via <span class="font-mono">POST /oauth/register</span> (RFC 7591). New clients get no access until you attach a group. Disable this once your client is connected.
+      <% else %>
+        New clients must be created here. Enable briefly to let a client (e.g. an MCP connector) register itself, then disable again.
+      <% end %>
+    </p>
+  </div>
+  <%= button_to dcr_on ? "Disable" : "Enable",
+        admin_dynamic_client_registration_path,
+        method: :patch,
+        params: {enabled: !dcr_on},
+        class: "shrink-0 rounded-md px-3 py-2 text-sm font-semibold text-white shadow-sm #{dcr_on ? "bg-amber-600 hover:bg-amber-500" : "bg-blue-600 hover:bg-blue-500"}" %>
+</div>
+
 <div class="mt-8 flow-root">
   <div class="-mx-4 -my-2 overflow-x-auto sm:-mx-6 lg:-mx-8">
     <div class="inline-block min-w-full py-2 align-middle sm:px-6 lg:px-8">