feat(web): keep deployed WebUI out of search-engine indexes

Searching "TsmusicBot" surfaced many deployed instances' WebUI URLs,
letting strangers walk into other people's control pages (issue #128).
Add defence-in-depth so crawlers stop indexing public deployments:

- send `X-Robots-Tag: noindex, nofollow` on every Express response
- serve `/robots.txt` with `User-agent: * / Disallow: /`
- add `<meta name="robots" content="noindex, nofollow">` to index.html,
  which also covers the /bot/<id> dedicated-link pages (same SPA shell)

These layers only prevent indexing; real protection stays with WebUI
auth and the reverse proxy. Document this in the README security section
and warn users not to post their WebUI link on public pages.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/web/robots.test.ts

@@ -0,0 +1,86 @@
+import { describe, it, expect } from "vitest";
+import express from "express";
+import request from "supertest";
+import fs from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+/**
+ * Search-engine hardening for issue #128: searching "TsmusicBot" surfaced a
+ * large number of deployed instances' WebUI URLs, letting strangers walk into
+ * other people's control pages. The fix is defence in depth — none of these
+ * layers is authentication (that's handled elsewhere), they just keep the
+ * public URL out of crawler indexes:
+ *
+ *   1. `X-Robots-Tag: noindex, nofollow` on EVERY response;
+ *   2. `GET /robots.txt` → `User-agent: * / Disallow: /`;
+ *   3. `<meta name="robots" content="noindex, nofollow">` in web/index.html.
+ *
+ * The header middleware and the /robots.txt route both live at the top of
+ * `createWebServer` in `server.ts`; this test asserts the exact behaviour we
+ * expect from them in isolation (the wiring inside server.ts is verified by
+ * code review / git diff, matching security-headers.test.ts).
+ */
+describe("search-engine hardening (issue #128 noindex)", () => {
+  function buildApp() {
+    const app = express();
+    // Mirrors the security-headers middleware in server.ts.
+    app.use((_req, res, next) => {
+      res.setHeader("X-Frame-Options", "DENY");
+      res.setHeader("Content-Security-Policy", "frame-ancestors 'none'");
+      res.setHeader("X-Robots-Tag", "noindex, nofollow");
+      next();
+    });
+    // Mirrors the public /robots.txt route in server.ts.
+    app.get("/robots.txt", (_req, res) => {
+      res.type("text/plain").send("User-agent: *\nDisallow: /\n");
+    });
+    app.get("/", (_req, res) => res.json({ ok: true }));
+    app.post("/api/session/login", (_req, res) => res.json({ ok: true }));
+    return app;
+  }
+
+  it("sets X-Robots-Tag: noindex, nofollow on GET responses", async () => {
+    const res = await request(buildApp()).get("/");
+    expect(res.status).toBe(200);
+    expect(res.headers["x-robots-tag"]).toBe("noindex, nofollow");
+  });
+
+  it("sets X-Robots-Tag on POST (API) responses too", async () => {
+    const res = await request(buildApp()).post("/api/session/login");
+    expect(res.headers["x-robots-tag"]).toBe("noindex, nofollow");
+  });
+
+  it("serves /robots.txt disallowing all crawlers", async () => {
+    const res = await request(buildApp()).get("/robots.txt");
+    expect(res.status).toBe(200);
+    expect(res.headers["content-type"]).toMatch(/text\/plain/);
+    expect(res.text).toContain("User-agent: *");
+    expect(res.text).toContain("Disallow: /");
+  });
+
+  it("still tags the /robots.txt response itself as noindex", async () => {
+    const res = await request(buildApp()).get("/robots.txt");
+    expect(res.headers["x-robots-tag"]).toBe("noindex, nofollow");
+  });
+});
+
+describe("frontend robots meta tag (issue #128 noindex)", () => {
+  const indexHtmlPath = path.resolve(
+    path.dirname(fileURLToPath(import.meta.url)),
+    "../../web/index.html"
+  );
+  const html = fs.readFileSync(indexHtmlPath, "utf-8");
+
+  const robotsMeta = html.match(
+    /<meta\s+name=["']robots["']\s+content=["']([^"']+)["']\s*\/?>/i
+  );
+
+  it("declares a robots meta tag", () => {
+    expect(robotsMeta).not.toBeNull();
+  });
+
+  it("marks the SPA shell noindex, nofollow (covers /bot/<id> dedicated links)", () => {
+    expect(robotsMeta?.[1]).toBe("noindex, nofollow");
+  });
+});

src/web/server.ts

@@ -77,12 +77,19 @@ export function createWebServer(options: WebServerOptions): WebServer {
     app.set("trust proxy", true);
   }
 
-  // Security headers: prevent the WebUI from being embedded in a third-party
-  // iframe (clickjacking defence). CSP frame-ancestors is the modern equivalent
-  // of X-Frame-Options; both are set for compatibility across browsers.
+  // Security headers:
+  //  • X-Frame-Options / CSP frame-ancestors — prevent the WebUI from being
+  //    embedded in a third-party iframe (clickjacking defence). CSP
+  //    frame-ancestors is the modern equivalent of X-Frame-Options; both are
+  //    set for compatibility across browsers.
+  //  • X-Robots-Tag — keep deployed instances out of search-engine indexes
+  //    (issue #128: searching "TsmusicBot" surfaced strangers' WebUI URLs).
+  //    Set on EVERY response so JSON/API responses and the SPA shell are all
+  //    covered; complements /robots.txt and the <meta name="robots"> tag.
   app.use((_req, res, next) => {
     res.setHeader("X-Frame-Options", "DENY");
     res.setHeader("Content-Security-Policy", "frame-ancestors 'none'");
+    res.setHeader("X-Robots-Tag", "noindex, nofollow");
     next();
   });
 
@@ -95,6 +102,13 @@ export function createWebServer(options: WebServerOptions): WebServer {
   const permissions = createPermissionStore(options.database.db);
 
   // ─── Public routes (no auth, no CSRF) ───────────────────────────────────
+  // Disallow every crawler (issue #128). Declared before the static SPA
+  // fallback so this wins over index.html for /robots.txt. Belt-and-braces
+  // with the X-Robots-Tag header above and the <meta name="robots"> tag.
+  app.get("/robots.txt", (_req, res) => {
+    res.type("text/plain").send("User-agent: *\nDisallow: /\n");
+  });
+
   app.get("/api/health", (_req, res) => {
     res.json({ status: "ok", version: "0.1.0" });
   });