feat(caddy): serve Audiobookshelf outside the guard chain

The Audiobookshelf phone app could never sign in over the public name. The guard
chain answered its /ping and /socket.io/ handshake with Anubis challenge HTML, so
the app evaluated 'isInit' in e.data on a non-object and reported "Failed to
contact server" while the container's request log recorded nothing at all. Anubis
demands a JavaScript proof-of-work and the authentik outpost works from a session
cookie: the chain is browser-only by construction, and a native client, a podcast
RSS reader or an OPDS client cannot pass it.

Audiobookshelf authenticates itself (local accounts plus authentik OIDC), so it
joins jelly/plex/*arr in the exempt set: the public site block dials the container
directly, keeping the X-Forwarded-Proto pin its OIDC origin check depends on, and
the @audiobooks route leaves the post-auth listener so one host has one owner. The
IdP rows that guarded the name go after this deploys, never before.

tests/e2e/specs/14-guard.spec.ts loses audiobooks as a guarded target and gains the
exemption assertion (app bytes to an anonymous client, the app's own 401, never
challenge HTML); tests/anubis.bats pins the route shape.

tests/e2e/specs/14-guard.spec.ts

@@ -6,21 +6,21 @@ import { expect, test } from "@playwright/test";
 // navigation would skip. These tests drive the browser as an UNAUTHENTICATED
 // user (the guest project has no storage state) and navigate directly to each
 // guarded vhost's root path: the guard must present authentik's flow UI in
-// place and must never leak app content. plex is the documented exemption —
-// PMS enforces its own auth — so it must be reachable unauthenticated with
-// zero challenge pages, proving the exemption is deliberate, not accidental.
+// place and must never leak app content. plex and audiobooks are the documented
+// exemptions — PMS and Audiobookshelf enforce their own auth — so they must be
+// reachable unauthenticated with zero challenge pages, proving the exemption is
+// deliberate, not accidental.
 
 const BASE = "https://www.franksdank.cloud";
 
 // Every vhost that Caddy routes through anubis:8923 (see dankclaw/Caddyfile).
 // Keep this list in lockstep with the Caddyfile: a new guarded vhost must be
 // added here or the guard-chain audit is incomplete.
-const GUARDED = ["www", "search", "romm", "files", "seerr", "grafana", "prometheus", "loki", "notebook", "audiobooks"];
+const GUARDED = ["www", "search", "romm", "files", "seerr", "grafana", "prometheus", "loki", "notebook"];
 
 // Markers that prove app content leaked past the guard to an anonymous user. Each
-// marker must be a string the APP renders that the gate itself could never print —
-// for audiobooks that is the Audiobookshelf login's own SSO button label (the app's
-// door), which authentik's gate never renders because it IS the IdP.
+// marker must be a string the APP renders that the gate itself could never print:
+// authentik's gate never renders an app's UI text, because it IS the IdP.
 const LEAK_MARKERS: Record<string, string[]> = {
   "search.franksdank.cloud": ["SearXNG preferences"],
   "romm.franksdank.cloud": ["RomM"],
@@ -29,7 +29,6 @@ const LEAK_MARKERS: Record<string, string[]> = {
   "grafana.franksdank.cloud": ["Dashboards"],
   "prometheus.franksdank.cloud": ["Expression Browser", "Graph"],
   "notebook.franksdank.cloud": ["Open Notebook"],
-  "audiobooks.franksdank.cloud": ["Sign in with Authentik"],
 };
 
 test("homepage root (/) is guarded for unauthenticated users", async ({ page }) => {
@@ -102,3 +101,54 @@ test("plex is exempt from the guard chain and serves PMS content unauthenticated
   const resp = await page.request.get("https://plex.franksdank.cloud/");
   expect(resp.status(), "PMS must answer directly, not error").toBeLessThan(500);
 });
+
+// Audiobookshelf is exempt by construction (2026-11-03). The phone app is a
+// non-browser client and solves no JS proof-of-work, so behind the guard chain it
+// was answered with challenge HTML wherever the socket `init` payload belongs —
+// every remote sign-in died as a TypeError on a non-object, and the container's
+// request log never saw a byte of it. ABS enforces its own auth (local accounts
+// plus authentik OIDC), exactly like PMS above. Asserted from the CLIENT side: the
+// endpoints an app must call before it holds any credential are answered by the
+// app, and the app's own 401 — not a 200 of HTML — is what an anonymous API call gets.
+test("audiobookshelf answers an unauthenticated app client with app bytes, not gate bytes", async ({
+  request,
+}) => {
+  const ABS = "https://audiobooks.franksdank.cloud";
+
+  // The probe every Audiobookshelf client sends first. The app answers it at the
+  // ROOT whatever ROUTER_BASE_PATH says, so this is the vhost's own behaviour.
+  const ping = await request.get(`${ABS}/ping`, { timeout: 30_000 });
+  expect(ping.status(), "/ping must be answered by the app").toBe(200);
+  expect(await ping.json(), "/ping must carry the app's own payload").toEqual({ success: true });
+
+  // The socket.io handshake. Engine.IO answers this in a machine format; the gate
+  // answered it in HTML, which is the defect this exemption exists to kill.
+  const sock = await request.get(`${ABS}/audiobookshelf/socket.io/?EIO=4&transport=polling`, {
+    timeout: 30_000,
+  });
+  expect(
+    await sock.text(),
+    "the socket handshake was answered by the gate, not Engine.IO",
+  ).not.toContain("Making sure you're not a bot");
+  expect(
+    sock.headers()["content-type"] ?? "",
+    "the socket handshake must not arrive as HTML",
+  ).not.toMatch(/text\/html/);
+
+  // An API call with no credential: the app refuses it. A gate would answer 200
+  // with HTML, so the status code alone distinguishes the two owners.
+  const api = await request.get(`${ABS}/audiobookshelf/api/libraries`, { timeout: 30_000 });
+  expect(api.status(), "the app's own auth gate must own unauthenticated API calls").toBe(401);
+});
+
+test("audiobookshelf login renders the app's own sign-in to an anonymous browser", async ({ page }) => {
+  test.setTimeout(120_000);
+  await page.goto("https://audiobooks.franksdank.cloud/audiobookshelf/login");
+  expect(new URL(page.url()).hostname).toBe("audiobooks.franksdank.cloud");
+
+  const body = await page.locator("body").innerText();
+  expect(body, "the bot challenge still fronts this vhost").not.toContain("Making sure you're not a bot");
+  expect(await page.locator('input[name="uidField"]').count(), "the IdP gate form must not front the app").toBe(0);
+  // The app answers, and its login screen carries SSO beside the local form.
+  await expect(page.getByText("Sign in with Authentik")).toBeVisible({ timeout: 90_000 });
+});