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>

apps/web/lib/env.ts

@@ -18,6 +18,17 @@ const envSchema = z.object({
   OIDC_CLIENT_ID_MCP: z.string().min(1),
   OIDC_AUDIENCE: z.string().min(1),
 
+  // Name of the IdP scope whose mapping emits `aud: <OIDC_AUDIENCE>`.
+  //
+  // Most IdPs (Authentik included) only evaluate a scope mapping when the
+  // client actually requests that scope. The MCP client learns which scopes
+  // to request from `scopes_supported` in our protected-resource metadata,
+  // so this name has to be advertised there or the mapping never runs and
+  // every token arrives without an `aud` claim (-> 401 "claim invalid: aud").
+  //
+  // Defaults to the `aud-<audience>` convention used in the README setup.
+  OIDC_AUDIENCE_SCOPE: z.string().min(1).optional(),
+
   // Database
   DATABASE_URL: z.string().url(),