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/anubis.bats

@@ -75,8 +75,10 @@ PY
 }
 
 @test "exempt vhosts never route through anubis (auth, plex, own-auth apps)" {
-    # The IdP login flow and git smart-HTTP clients cannot solve challenges;
-    # these vhosts bypass the chain by construction.
+    # The IdP login flow, git smart-HTTP clients, and any non-browser client (the
+    # Audiobookshelf phone app, podcast RSS readers) cannot solve a challenge, so
+    # services that enforce their own complete auth bypass the chain by
+    # construction — the gate would answer their API calls with HTML.
     local block
     block=$(awk '/^auth\.franksdank\.cloud/,/^}/' "$CADDY")
     echo "$block" | grep -q 'reverse_proxy authentik-server:9000'
@@ -84,4 +86,11 @@ PY
     block=$(awk '/^plex\.franksdank\.cloud/,/^}/' "$CADDY")
     echo "$block" | grep -q '192\.168\.0\.235:32400'
     ! echo "$block" | grep -q anubis
+    block=$(awk '/^audiobooks\.franksdank\.cloud/,/^}/' "$CADDY")
+    echo "$block" | grep -q '192\.168\.0\.235:8000'
+    ! echo "$block" | grep -q anubis
+    # The exemption is total: the post-auth listener must not still carry an
+    # audiobooks route. A leftover handle there is a second owner for one host,
+    # and its X-Forwarded-Proto pin would silently disagree with the edge.
+    ! grep -q '@audiobooks host audiobooks.franksdank.cloud' "$CADDY"
 }