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.

tests/integration/doctor-report-sections.test.ts

@@ -317,6 +317,7 @@ describe("doctor-report — module enable/disable gating", () => {
     insights: true,
     medications: true,
     doctorReport: true,
+    mcp: true,
   } as const;
 
   it("includes every module's data when all modules are enabled", async () => {