feat: add RFC 8707 resource indicator via resource config (#161)

* feat: add RFC 8707 resource indicator via resource config

`Config.Resource` is an alternative to `audience` for IdPs that implement
RFC 8707. `buildAuthURL` sends it as the `resource` parameter on the
authorization request, before `extraAuthParams` so an operator-configured
`resource` key there cannot override or duplicate it. `exchangeTokens`
repeats it on both the `authorization_code` and `refresh_token` grants
(RFC 8707 §2.2), so a refreshed token keeps the audience binding of the
original grant.

Only one resource is accepted. RFC 8707 permits a list, but no provider
that supports the parameter handles a list usefully: one rejects any
request naming more than one resource with HTTP 400 `invalid_target`;
node-oidc-provider accepts several and issues an opaque token bound to
none of them.

`Config.Validate()` requires an absolute URI of at most 256 characters
with no fragment, wildcard, or control characters. `http`/`https` values
go through `isValidSecureURL`, so HTTPS is required outside loopback;
other schemes (`urn:`) pass as-is.

When `resource` is set and `audience` is not, `audience` defaults to the
resource for access-token validation; `strictAudienceValidation` applies
the same way whichever of the two is set. That default is validation-only.
A new `TraefikOidc.explicitAudience` mirrors `Config.Audience` verbatim,
and `buildAuthURL` keys the non-standard `audience` request parameter (the
Auth0 custom-API pattern) on it rather than on `audience`, so a
resource-derived default is never sent as an unsolicited `audience`
parameter alongside `resource` — a combination that misbehaves against
providers such as Ory Hydra, which give `audience` a different meaning.
`audience_test.go` and `url_helpers_ultra_test.go` set `audience` on the
struct directly, bypassing `New()`, and now set `explicitAudience` too.

docs/CONFIGURATION.md carries only the reference-table row and a short
pointer; the new docs/RFC8707.md holds the request behaviour, the
single-resource rationale, the `resource`/`audience` relationship, and a
provider-support table from interop testing: node-oidc-provider supports
it; Zitadel accepts and silently ignores it (`oidc.AuthRequest` has no
`resource` field, confirmed against a live instance); Authentik returns
`invalid_target`; Keycloak has no support through 26.6; Ory Hydra/Fosite
uses its own `audience` parameter; Azure AD/Entra ID v2 is reported to
reject `resource` combined with v2-style `scope`; Auth0 and AWS Cognito
claims are unverified. README.md, docs/index.html, and .traefik.yml each
point at docs/RFC8707.md instead of repeating it, and the comments on
`Config.Resource`, in main.go, and in url_helpers.go defer to it as well.

rfc8707_resource_test.go covers the authorization-URL parameter (set,
unset, query-bearing value percent-encoded rather than merged,
`extraAuthParams` unable to override), both grants sending `resource` in
the form body, the `Config.Validate()` rules table, audience defaulting
through `New()`, and a regression pair: a resource-derived audience
default is not sent as `audience`, while an explicitly configured one
still is.

* fix: accept Resource in place of Audience for bearer-auth validation

`NewWithContext` rejected `EnableBearerAuth=true` whenever
`config.Audience` was empty, even though `t.audience` already falls back
to `config.Resource` in that case. The check now fails only when both
`Audience` and `Resource` are empty, and the error names both fields.
`TestStartupValidation_BearerAcceptsResourceInPlaceOfAudience` covers the
`Resource`-only configuration.

`docs/RFC8707.md` links the `audience` parameter note to
`docs/AUTH0_AUDIENCE_GUIDE.md` instead of restating it, and drops the
provider table's GitHub row and the interop-testing paragraph, which are
covered elsewhere.

docs/RFC8707.md

@@ -0,0 +1,54 @@
+# RFC 8707 Resource Indicators
+
+`resource` ([Config.Resource](../settings.go)) is an alternative to `audience` for IdPs that
+implement [RFC 8707](https://datatracker.ietf.org/doc/html/rfc8707). See
+[docs/CONFIGURATION.md](CONFIGURATION.md#audience-validation) for the config reference; this
+document covers exactly what request is made and which providers support it.
+
+## What the plugin sends
+
+- One `resource` parameter on the authorization request (`buildAuthURL`), added before
+  `extraAuthParams` so an operator-configured `resource` key there cannot override or
+  duplicate it.
+- The same `resource` parameter on both the `authorization_code` and `refresh_token` grants
+  (`exchangeTokens`), per RFC 8707 §2.2 — so a refreshed token keeps the same audience binding
+  as the original grant.
+
+Only a single resource is supported, not a list. RFC 8707 allows a server to accept multiple
+`resource` values and scope the issued token to a subset of them, but no provider found to
+support the parameter at all handles that usefully: one provider tested rejects a request
+naming more than one resource with HTTP 400 `invalid_target`; node-oidc-provider accepts
+multiple resources but issues an opaque access token bound to none of them. A single value
+avoids both failure modes.
+
+## Relationship to `audience`
+
+`resource` and `audience` both feed the same access-token audience check —
+`strictAudienceValidation` applies identically to either — but they produce different requests
+on the wire:
+
+- `audience`, when explicitly configured, is sent as a non-standard `audience` request
+  parameter (see [docs/AUTH0\_AUDIENCE\_GUIDE.md](AUTH0_AUDIENCE_GUIDE.md)).
+- `resource` is sent as the RFC 8707 `resource` parameter instead.
+- If `resource` is set and `audience` is not, the expected-audience check defaults to the
+  resource value, but this is validation-only: it does not cause an `audience` parameter to be
+  sent. Setting both is allowed — e.g. as a hedge against a provider that ignores `resource` —
+  in which case both parameters are sent.
+
+## Provider support
+
+Verified 2026-09-02 (Zitadel reverified separately against a live instance). RFC 8707 requires
+a server that does not support `resource` to *ignore* it, but not every implementation does —
+verify against your specific provider and version before relying on it.
+
+| Provider | Support | Notes |
+|---|---|---|
+| node-oidc-provider | Supported | Via its `resourceIndicators` feature. |
+| Zitadel | Ignored, not rejected | `oidc.AuthRequest` has no `resource` field; confirmed against a live instance — `/oauth/v2/authorize` returns the same `302` with or without `resource`, including a spec-invalid value. |
+| Authentik | Rejected | Returns `invalid_target` ("Resource indicators are not supported"). |
+| Keycloak | Not supported (through 26.6) | Its own guidance is an audience mapper plus `scope` instead ([keycloak/keycloak#41526](https://github.com/keycloak/keycloak/issues/41526)). |
+| Ory Hydra / Fosite | Not implemented | Uses its own non-standard `audience` request parameter instead. |
+| Azure AD / Entra ID v2.0 (MSAL) | Reported to reject | When combined with v2-style `scope`, rather than ignoring it ([writeup](https://www.groff.dev/blog/azure-entra-id-mcp-server-authentication-incompatibilities)). Do not enable against an Entra ID v2 tenant without confirming. The v1.0 endpoint has a separate, pre-RFC `resource` parameter with similar but not identical semantics. |
+| Auth0 | Not verified | Documents a Resource Parameter Compatibility Profile mapping `resource` → `audience` ([Auth0 docs](https://auth0.com/ai/docs/mcp/guides/resource-param-compatibility-profile)). |
+| AWS Cognito | Not verified | Announced Oct 2025 for user-pool authorization-code/implicit grants ([announcement](https://aws.amazon.com/about-aws/whats-new/2025/10/amazon-cognito-resource-indicators-protection-oauth-2-0-resources)). |
+| Okta, Google, GitLab | Unverified | No confirming documentation found either way. |