feat(mcp): serve the read-only tools over remote Streamable HTTP at /mcp

Bridge the existing transport-agnostic MCP server factory to a Next.js
route at /mcp using the SDK's Web-standard Streamable HTTP transport, so a
remote client can connect with a manually-minted hlk_ Bearer token. The
same tool/resource layer the local stdio transport serves is reused
verbatim — no duplicated tool logic.

The route owns four gates in front of the transport:

- Auth: Authorization: Bearer hlk_… resolved by the same validator the
  HTTP edge uses, so revoked/expired/scope semantics never drift. A cookie
  is never consulted, so the cookie-only admin boundary stays unreachable
  here regardless of token scope.
- Rate limit: the Postgres limiter keyed by the <userId>:<tokenId> binding
  (mcp:<binding>), so a single shared credential cannot drain the account.
- Module gate: a new opt-in mcp module, OFF by default. With it off the
  endpoint answers 404 and reveals nothing — a self-hoster who never opts
  in gets zero new exposure. The Modules settings hub renders the toggle.
- Transport: a fresh stateless server + transport per request.

Read-only by construction (only read tools are registered) and same-origin
by construction (no CORS headers). The proxy lets /mcp skip the page cookie
redirect since it authenticates by header, mirroring the webhook surfaces.

src/proxy.ts

@@ -86,6 +86,16 @@ function isPublicPath(pathname: string): boolean {
   if (pathname === "/onboarding" || pathname.startsWith("/onboarding/")) {
     return true;
   }
+  // v1.22.0 — the remote MCP endpoint authenticates by `Authorization:
+  // Bearer hlk_…`, never by a session cookie, so it must skip the page
+  // cookie-redirect (mirroring the webhook / `/api/c/` header-authed
+  // surfaces). The route handler resolves the Bearer token itself, gates on
+  // the off-by-default `mcp` module, and rate-limits per credential. Exact
+  // `/mcp` plus `/mcp/` only — a `/mcp-something` page can never inherit
+  // the bypass.
+  if (pathname === "/mcp" || pathname.startsWith("/mcp/")) {
+    return true;
+  }
   if (WELL_KNOWN_PUBLIC_PATHS.has(pathname)) {
     return true;
   }