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

docs/decisions/README.md

@@ -10,3 +10,4 @@ Each file is one decision. Newest decisions get the next number.
 | [0001](0001-opaque-vs-jwt-access-tokens.md) | Access tokens are opaque (not JWT); resource servers use introspection |
 | [0002](0002-device-authorization-grant.md) | CLI/agent auth uses the OAuth 2.0 Device Authorization Grant (RFC 8628) |
 | [0003](0003-dynamic-client-registration.md) | Dynamic Client Registration (RFC 7591), runtime-gated + default-deny |
+| [0004](0004-resource-indicators.md) | Resource Indicators (RFC 8707) bind token audience; pass-through validation |