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

db/seeds.rb

@@ -7,3 +7,45 @@
 #   ["Action", "Comedy", "Drama", "Horror"].each do |genre_name|
 #     MovieGenre.find_or_create_by!(name: genre_name)
 #   end
+
+# --- OAuth clients for CLI / agent access and token introspection ---------------
+#
+# These support the Device Authorization Grant (RFC 8628) and RFC 7662 token
+# introspection. See docs/decisions/0002-device-authorization-grant.md.
+
+admins = Group.find_by(admin: true)
+
+# Public client (no secret, PKCE) used by CLIs and agents via the device flow.
+# Ships with a well-known client_id so tools can hard-code it.
+cli = Application.find_or_create_by!(client_id: "clinch-cli") do |app|
+  app.name = "Clinch CLI"
+  app.slug = "clinch-cli"
+  app.app_type = "oidc"
+  app.is_public_client = true
+  app.active = true
+end
+
+# Grant the CLI to the admins group by default (device flow enforces
+# Application#user_allowed?). Adjust to taste.
+if admins && cli.allowed_groups.exclude?(admins)
+  cli.allowed_groups << admins
+  puts "Seeded 'clinch-cli' public client (allowed group: #{admins.name})."
+end
+
+# Confidential client that resource servers (e.g. c2a2) use to authenticate to
+# the introspection endpoint. The secret is only shown once, on creation.
+unless Application.exists?(client_id: "c2a2-introspection")
+  secret = SecureRandom.urlsafe_base64(48)
+  Application.create!(
+    name: "c2a2 (introspection caller)",
+    slug: "c2a2-introspection",
+    client_id: "c2a2-introspection",
+    client_secret: secret,
+    app_type: "oidc",
+    active: true
+  )
+  puts "Seeded 'c2a2-introspection' confidential client:"
+  puts "  client_id:     c2a2-introspection"
+  puts "  client_secret: #{secret}"
+  puts "  Store these in c2a2 now — the secret is hashed and cannot be recovered."
+end