fix: advertise the audience scope so tokens actually carry aud

The OAuth path to /api/mcp has never worked end to end. Every access token
arrived without an `aud` claim and jwt.ts rejected it with
"claim invalid: aud" (401), even though the handshake, consent and PKCE all
succeeded. Only the CLI HMAC path worked, because cli-token.ts sets the
audience itself — which is why this went unnoticed.

Cause: Authentik evaluates a scope mapping only when the client REQUESTS
that scope by name. An MCP client learns which scopes to request from
`scopes_supported` in our RFC 9728 protected-resource metadata, and we only
advertised openid/profile/email. So the `aud-shared-memory` mapping was
attached to the provider but never evaluated.

Advertise the audience scope in that metadata. Name is derived as
`aud-<OIDC_AUDIENCE>` to match the README convention, overridable with the
new optional OIDC_AUDIENCE_SCOPE for deployments that named it differently.

Also documents that Claude Code's RFC 8707 `resource` parameter is ignored
by Authentik 2026.5, so it cannot be relied on for audience binding.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.env.example

@@ -35,6 +35,12 @@ OIDC_CLIENT_ID_WEB=replace-me
 OIDC_CLIENT_SECRET_WEB=replace-me
 OIDC_CLIENT_ID_MCP=replace-me
 OIDC_AUDIENCE=shared-memory
+# Scope whose IdP mapping emits `aud: <OIDC_AUDIENCE>`. Advertised in
+# /.well-known/oauth-protected-resource so MCP clients request it — without
+# that, Authentik never evaluates the mapping and every token 401s with
+# "claim invalid: aud". Defaults to aud-<OIDC_AUDIENCE>; set only if you
+# named the scope mapping something else.
+#OIDC_AUDIENCE_SCOPE=aud-shared-memory
 
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