fix(security): require proof that x-9r-real-ip came from the socket (GHSA-pjm4-8fpg-f9p6)

x-9r-real-ip and the Host fallback were trusted from client-controlled
headers whenever custom-server.js was not in the request path (npm run
start, start:bun), letting a remote caller pose as local to skip API key
auth and reach LOCAL_ONLY_PATHS (/api/mcp/*, /api/tunnel/enable,
/api/auth/reset-password).

custom-server.js now generates a per-process secret at boot and stamps it
as x-9r-peer-token on every request it sanitizes. hasTrustedPeerHeaders()
(src/lib/auth/trustedPeer.js) gates trust in x-9r-real-ip on that secret;
otherwise the guard falls back to Host only in development, and fails
closed in production. Same gate on loginLimiter.getClientIp() so a spoofed
header cannot rotate the login lockout bucket.

Also: fix isLoopbackHostname for IPv6 (::1, ::ffff:127.0.0.1) which the
old split(":")[0] reduced to empty string; route npm run start /
start:bun through custom-server.js (postbuild copies it into
.next/standalone, build-cli.js fails without it) so documented deployments
keep passwordless local access.

tests/unit/standalone-assets.test.js

@@ -37,6 +37,17 @@ describe("standalone build assets", () => {
       .toBe("static asset");
   });
 
+  // Without the wrapper beside server.js nothing can prove a request is local.
+  it("copies the request-sanitizing server wrapper into the standalone output", () => {
+    const projectRoot = createBuildFixture(".next");
+    writeFileSync(join(projectRoot, "custom-server.js"), "wrapper");
+
+    copyStandaloneAssets({ projectRoot, distDir: ".next" });
+
+    expect(readFileSync(join(projectRoot, ".next", "standalone", "custom-server.js"), "utf8"))
+      .toBe("wrapper");
+  });
+
   it("does not modify workspace-traced CLI builds", () => {
     const projectRoot = createBuildFixture(".next-cli-build");
     const previousMode = process.env.NEXT_TRACING_ROOT_MODE;