Add RFC 8707 Resource Indicators (audience binding)

Clients can name a target resource server via the `resource` parameter at the
authorization and device-authorization endpoints. Clinch binds it to the issued
token as its audience and reports it at introspection as `aud`, so a token minted
for one API cannot be replayed against another that also trusts clinch.

- `resource` is validated syntax-only (absolute URI, no fragment) — pass-through,
  no resource registry; the resource server enforces the audience on introspection
- threaded from authorize / device_authorization onto the auth/device code, then
  the access and refresh tokens, and carried across refresh rotation
- invalid values are rejected with error=invalid_target
- introspection `aud` is the bound resource, falling back to the client_id
- ADR 0004 records the pass-through-vs-registry decision

Completes the clinch-side OAuth surface for MCP connectors (with DCR +
introspection); Protected Resource Metadata (RFC 9728) lives on the resource server.

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

db/migrate/20260719000003_add_resource_to_oidc_tokens.rb

@@ -0,0 +1,12 @@
+class AddResourceToOidcTokens < ActiveRecord::Migration[8.1]
+  # RFC 8707 Resource Indicators: the audience (target resource server) a token
+  # is bound to. Threaded from the authorize / device_authorization request
+  # through the code and carried across refresh rotation onto the access token,
+  # where introspection reports it as `aud`.
+  def change
+    add_column :oidc_authorization_codes, :resource, :string
+    add_column :oidc_device_codes, :resource, :string
+    add_column :oidc_access_tokens, :resource, :string
+    add_column :oidc_refresh_tokens, :resource, :string
+  end
+end