fix: serve RFC 9728 path-suffixed metadata, document connector redirect URIs

Two separate discovery footguns, both found while debugging an Authentik
"Redirect URI Error" on a claude.ai custom connector.

RFC 9728 §3.1 puts the metadata for a resource identified by
`https://host/api/mcp` at `/.well-known/oauth-protected-resource/api/mcp`.
Only the root form was served, so clients that derive the metadata URL from
the MCP endpoint URL — rather than reading `resource_metadata` off our 401 —
got Next.js's HTML 404 and failed discovery with a JSON parse error.

Add a `[...path]` route serving the same document with `resource` naming the
suffixed identifier (§3.3 has the client compare it as an exact string, so
echoing the bare origin would be rejected). The document body moves to
`lib/auth/resource-metadata.ts` so the two routes cannot drift apart on
`scopes_supported` — a divergence there costs you the `aud` claim or the
refresh token. Paths are allowlisted rather than wildcarded so this cannot
advertise resources the app does not serve. `buildWwwAuthenticate()` still
points at the root URL; this change is purely additive.

Separately, the redirect URIs an MCP provider needs depend on how clients
reach it: a loopback URI for the CLI, `https://claude.ai/api/mcp/auth_callback`
for a claude.ai custom connector. Registering only the former is what produces
the "Redirect URI Error" page, and a portless `http://localhost/callback`
entry matches nothing the CLI sends. Document both, keyed on the literal
error text, and note that DCR is enterprise-gated on Authentik so these are
hand-registered on a FOSS instance.

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

apps/web/lib/auth/resource-metadata.ts

@@ -0,0 +1,73 @@
+import { env } from "@/lib/env";
+import { mcpIssuer } from "@/lib/auth/jwt";
+
+/**
+ * The protected-resource metadata document, RFC 9728 §2.
+ *
+ * Shared by both metadata routes — the root `/.well-known/oauth-protected-
+ * resource` and the path-suffixed `/.well-known/oauth-protected-resource/
+ * <resource path>` form of §3.1 — because the two documents differ ONLY in
+ * the `resource` identifier they describe. Anything else drifting between
+ * them is a bug: a client that discovers us through the suffixed URL would
+ * be told to request a different scope set than one that follows the
+ * `WWW-Authenticate: resource_metadata=...` header, and whichever of the two
+ * lost the audience scope would hand back tokens with no `aud` claim.
+ */
+export interface ResourceMetadata {
+  resource: string;
+  authorization_servers: string[];
+  scopes_supported: string[];
+  bearer_methods_supported: string[];
+  resource_documentation: string;
+}
+
+/**
+ * The public origin, with any trailing slash stripped.
+ *
+ * `resource` values are compared as exact strings by clients (RFC 9728 §3.3),
+ * so `https://host/` and `https://host` are not interchangeable — PUBLIC_URL
+ * is written both ways in the wild and only the stripped form is emitted.
+ */
+export function publicOrigin(): string {
+  return env().PUBLIC_URL.replace(/\/$/, "");
+}
+
+/**
+ * Build the metadata document for `resource`.
+ *
+ * The caller supplies the resource identifier because it depends on which
+ * URL the document was fetched from; everything else is deployment config.
+ */
+export function buildResourceMetadata(resource: string): ResourceMetadata {
+  // The audience scope MUST be advertised. Authentik only evaluates a scope
+  // mapping when the client requests that scope by name, and the client only
+  // learns scope names from this document. Omit it and every access token
+  // arrives without `aud`, which jwt.ts rejects as "claim invalid: aud".
+  const audienceScope =
+    env().OIDC_AUDIENCE_SCOPE ?? `aud-${env().OIDC_AUDIENCE}`;
+
+  // Same mechanism as the audience scope, different consequence: a client
+  // only requests `offline_access` if it sees the name here, and without
+  // that request the IdP returns no refresh token — so the client cannot
+  // renew and the user gets kicked back to an interactive login whenever
+  // the access token expires.
+  //
+  // Opt-in, because the IdP needs a matching scope mapping; advertising one
+  // it doesn't offer can fail the whole authorization request.
+  const scopes = ["openid", "profile", "email", audienceScope];
+  if (env().OIDC_OFFLINE_ACCESS) scopes.push("offline_access");
+
+  return {
+    resource,
+    // The MCP application's issuer, which is not necessarily the Web UI's —
+    // see mcpIssuer(). Advertising the wrong one sends clients to a discovery
+    // document whose tokens this endpoint will then reject on `iss`.
+    authorization_servers: [mcpIssuer()],  // as configured, slash and all
+    scopes_supported: scopes,
+    bearer_methods_supported: ["header"],
+    // Documentation lives at the site root regardless of which resource this
+    // document describes, so it is always derived from the public origin and
+    // not from `resource`.
+    resource_documentation: `${publicOrigin()}/`,
+  };
+}