fix(career): bound the prepare request; validate the setlist (PR #971 review)

Both CodeRabbit findings were right.

1. A HUNG PREPARE COULD BLOCK THE GIG FOREVER.

   `await fetch(...)` only rejects on a network ERROR. A server that accepts the
   connection and then never answers hangs indefinitely — and the gig would never
   start. That makes this optimisation the exact thing the PR promises it can
   never be: the reason you cannot play.

   The request is now bounded by an AbortController (PREPARE_TIMEOUT_MS, generous
   because unpacking a setlist is real work — but a CEILING, not a wait). Past it
   we start the gig and let the first play extract lazily, as it always did. The
   Play button is restored in a `finally`, so a timeout cannot strand the poster
   on "Preparing set…" with Play disabled — which would have been the same bug
   wearing a different hat.

2. THE `songs` BODY WAS UNVALIDATED.

   A str is iterable: "abc" would have prepared three one-character "songs". And
   the endpoint unpacks zips, so an arbitrary caller could ask for unbounded work.
   Now list-only, string entries, blanks dropped, capped at MAX_GIG_SONGS.

Tests: the fetch is abortable and the button is re-enabled on EVERY path
including the abort; non-list bodies, non-string/blank entries, and an
oversized setlist. 50 career tests, JS 5/5, eslint clean.

tests/js/gig_song_end.test.js

@@ -75,3 +75,24 @@ test('the prepare route degrades instead of failing', () => {
         'a host without the library resolvers must degrade, not 500 — pre-extraction ' +
         'is an optimisation and can never be why a gig will not start');
 });
+
+// ── the prepare must never be able to BLOCK the gig (CodeRabbit, #971) ──────
+//
+// A bare `await fetch(...)` only rejects on a network error. A server that
+// accepts the connection and then never answers hangs forever — and the gig
+// would never start. That would make this optimisation the exact thing it
+// promises never to be: the reason you cannot play.
+
+test('the prepare fetch is bounded — a hung server cannot block the gig', () => {
+    const fn = extractBlock(CAREER, 'async function prepareGigSongs(');
+    assert.match(fn, /AbortController/, 'the request must be abortable');
+    assert.match(fn, /setTimeout\([\s\S]{0,40}abort\s*\(\s*\)/,
+        'a hung request must be aborted, not awaited forever');
+    assert.match(fn, /signal:\s*ctrl\.signal/, 'the signal must actually be passed to fetch');
+    assert.match(fn, /clearTimeout/, 'the timer must be cleared on the happy path');
+    assert.match(CAREER, /const\s+PREPARE_TIMEOUT_MS\s*=\s*\d+/, 'the ceiling must be named');
+    // The button must be restored however we leave — otherwise a timeout strands
+    // the poster on "Preparing set…" with Play disabled: unplayable.
+    assert.match(fn, /finally\s*\{[\s\S]{0,220}btn\.disabled\s*=\s*false/,
+        'the Play button must be re-enabled on EVERY path, including the abort');
+});