fix: retry apibay search when it answers with its no-results sentinel

[zanmlakar]

What and why

apibay's q.php caches search results per exact URL, and a query can get stuck with the single "No results returned" placeholder on one URL form while the alternate form answers fine. torlink always sends the bare form (q.php?q=…), so a poisoned query renders an empty TPB column in every tab even though the index has plenty. This retries once with &cat=0 when the sentinel comes back: the poisoned case turns into results, a genuinely empty search costs one extra request and still comes back empty, and browses (precompiled feeds) never retry.

Measured against the live API today (2026-08-14), stable across repeated runs ~40 minutes apart:

query	q.php?q=…	q.php?q=…&cat=0
metallica	sentinel (5/5)	100 rows (4/4)
abba	sentinel (3/3)	100 rows
pink floyd	sentinel (3/3)	100 rows
dune	100 rows (3/3)	sentinel (3/3)
daft punk · taylor swift · oppenheimer · breaking bad · ubuntu	100 rows	100 rows

Whichever form is poisoned, the other answered in every observed case — hence retry-on-sentinel rather than swapping forms wholesale (dune shows a swap would just move the hole). Live through the patched module: tpbMovies.search("metallica") goes from 0 rows to 9, "abba" from 0 to 8.

The four new tests pin the retry URL, the single-request happy path, the both-forms-empty case, and that browses never retry.

Found while verifying #141's TPB music source against the live API; it affects Movies and TV identically today, so it ships separately as its own concern.

Checklist

• npm run typecheck is clean
• npm test passes
• New logic has a test (vitest; mock node built-ins for platform code)
• If I added a key, I updated both HELP_GROUPS and footerHints in src/ui/keymap.ts
• If I added a Store field, I updated makeStore in scripts/render-previews-impl.tsx
• OS-touching code works on Windows, macOS, and Linux
• One concern, with a Conventional Commits title (feat: / fix: / docs: / chore:)

[baairon]

Merged, thank you Žan, and nice catch on that sentinel. On the music tab in #141, torlink is in maintenance mode right now so I'm not taking on new features, but I appreciate the work you put into it.

[zanmlakar]

Thanks for the review and merge!