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

docs/decisions/0003-dynamic-client-registration.md

@@ -0,0 +1,55 @@
+# 0003 — Dynamic Client Registration (RFC 7591), runtime-gated
+
+**Status:** Accepted · **Date:** 2026-07-19
+
+## Decision
+
+Clinch supports **OAuth 2.0 Dynamic Client Registration (RFC 7591)** at
+`POST /oauth/register`, so clients (notably MCP connectors like Claude) can register
+themselves instead of being hand-created. It is **off by default** and toggled at runtime
+by an admin from the Applications page. A newly registered client is **default-deny**: it
+has no `allowed_groups` until an admin attaches one.
+
+We also serve the **RFC 8414** metadata alias at
+`/.well-known/oauth-authorization-server` (the OIDC discovery document is a superset), and
+advertise `registration_endpoint` only while registration is enabled.
+
+## Context
+
+MCP connectors expect to self-register via anonymous DCR rather than being pre-provisioned.
+But open registration is a real risk: anyone could register a legitimate-looking client and
+attempt **consent phishing** — luring a user to approve it, then holding a token that acts
+as that user against any resource server that trusts clinch tokens (via introspection, see
+[0001](0001-opaque-vs-jwt-access-tokens.md)).
+
+Two controls make this safe:
+
+1. **Runtime window, not always-on.** DCR is a toggle (persisted `Setting`, admin UI), so
+   the operator opens it briefly, lets the client register, attaches a group, and closes it
+   again. The `CLINCH_DCR_ENABLED` env var is only a bootstrap default when the setting is
+   unset. Default is off.
+2. **Default-deny for new clients.** Clinch's authorize flow already gates on
+   `Application#user_allowed?` (group membership), evaluated *before* the consent screen
+   renders. A group-less registered client therefore can't show any user an approve button —
+   the consent-phishing path dead-ends until an admin explicitly grants a group. ForwardAuth
+   services are gated by the user's session cookie, not client tokens, so DCR doesn't widen
+   that surface at all.
+
+## Implementation notes
+
+- `OidcRegistrationController#create`: validates `token_endpoint_auth_method`
+  (none/client_secret_basic/client_secret_post), `grant_types`
+  (authorization_code/refresh_token), `response_types` (code), and `redirect_uris`
+  (https anywhere; http only for loopback). Creates a public or confidential `Application`
+  with `require_pkce: true`, returns the RFC 7591 response (client_secret once, for
+  confidential clients).
+- `Setting` is a small key/value store; `Application.dynamic_registration_enabled?` reads it
+  with the env var as fallback. Admin toggle: `Admin::DynamicClientRegistrationController`.
+
+## Consequences
+
+- MCP connectors can self-register when the window is open, then operate normally once an
+  admin grants a group.
+- No always-on anonymous registration surface; the risky window is short and operator-driven.
+- Remaining MCP pieces (resource indicators RFC 8707, protected-resource metadata RFC 9728 on
+  the resource server) are separate, smaller follow-ups.