feat: private hostname guard for authenticated/private mode

Reject requests from unrecognised Host headers when running
authenticated/private. Adds server middleware, CLI `allowed-hostname`
command, config-schema field, and prompt support for configuring
allowed hostnames during onboard/configure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/__tests__/private-hostname-guard.test.ts

@@ -0,0 +1,56 @@
+import { describe, expect, it } from "vitest";
+import express from "express";
+import request from "supertest";
+import { privateHostnameGuard } from "../middleware/private-hostname-guard.js";
+
+function createApp(opts: { enabled: boolean; allowedHostnames?: string[]; bindHost?: string }) {
+  const app = express();
+  app.use(
+    privateHostnameGuard({
+      enabled: opts.enabled,
+      allowedHostnames: opts.allowedHostnames ?? [],
+      bindHost: opts.bindHost ?? "0.0.0.0",
+    }),
+  );
+  app.get("/api/health", (_req, res) => {
+    res.status(200).json({ status: "ok" });
+  });
+  app.get("/dashboard", (_req, res) => {
+    res.status(200).send("ok");
+  });
+  return app;
+}
+
+describe("privateHostnameGuard", () => {
+  it("allows requests when disabled", async () => {
+    const app = createApp({ enabled: false });
+    const res = await request(app).get("/api/health").set("Host", "dotta-macbook-pro:3100");
+    expect(res.status).toBe(200);
+  });
+
+  it("allows loopback hostnames", async () => {
+    const app = createApp({ enabled: true });
+    const res = await request(app).get("/api/health").set("Host", "localhost:3100");
+    expect(res.status).toBe(200);
+  });
+
+  it("allows explicitly configured hostnames", async () => {
+    const app = createApp({ enabled: true, allowedHostnames: ["dotta-macbook-pro"] });
+    const res = await request(app).get("/api/health").set("Host", "dotta-macbook-pro:3100");
+    expect(res.status).toBe(200);
+  });
+
+  it("blocks unknown hostnames with remediation command", async () => {
+    const app = createApp({ enabled: true, allowedHostnames: ["some-other-host"] });
+    const res = await request(app).get("/api/health").set("Host", "dotta-macbook-pro:3100");
+    expect(res.status).toBe(403);
+    expect(res.body?.error).toContain("please run pnpm paperclip allowed-hostname dotta-macbook-pro");
+  });
+
+  it("blocks unknown hostnames on page routes with plain-text remediation command", async () => {
+    const app = createApp({ enabled: true, allowedHostnames: ["some-other-host"] });
+    const res = await request(app).get("/dashboard").set("Host", "dotta-macbook-pro:3100");
+    expect(res.status).toBe(403);
+    expect(res.text).toContain("please run pnpm paperclip allowed-hostname dotta-macbook-pro");
+  });
+});