fix(english/fucknovelpia): throw on rate-limited pages instead of empty chapters

[RibatTRW]

Fix

Closes #2540. fucknovelpia.com intermittently serves anti-scrape pages instead of chapter content (nginx 403, a custom 429 "Blocked" page, a Cloudflare 429 page) — none containing the .reader wrapper. parseChapter ignored the HTTP status and returned '' when .reader was missing, so a throttled request rendered as a false empty chapter. The reported chapter is fine: on a 200 it serves the full ~13.5k chars. Reproduced pre-fix: a burst of 12 rapid calls turned a 429 into a zero-length chapter.

• All fetches go through a shared helper sending browser-like headers, waiting out one 429 (honoring Retry-After, capped at 10s), and throwing with the HTTP status on refusal.
• parseChapter throws when a 200 page has no reader markup instead of returning ''.
• Version 1.1.1 → 1.1.2.

Verification

Burst re-test post-fix: 9 calls OK (retried 429s recovered to 200), 3 honest HTTP 429 throws, 0 silent empties. The reported chapter returns 13,577 chars with neighbors intact; parseNovel intact (1377 chapters with summary and cover). check:plugin passes all four steps; eslint, prettier, and build:compile clean on the touched file (repo-wide lint/format failures are pre-existing in untouched files).

Review follow-up

An AI review of this PR found one defect, fixed in the second commit:

• Fixed: the site's real 429 responses (Cloudflare) have no Retry-After header. Number(null) is 0, so the retry fired immediately instead of after the intended 2s fallback. Across three live trials, an immediate retry never recovered, while a 2–5s wait sometimes did. The header is now parsed with parseInt, so a missing or non-numeric value falls back to 2s. Verified with a stubbed 429 response and no header: the wait before the retry went from 0 ms to 2000 ms.
• No other defects confirmed. Every fetch path goes through fetchHtml, the live chapter/novel/search/updates pages return 200 with the expected markup, and the version bump to 1.1.2 still covers the unreleased change.

Ran: npm run check:plugin -- plugins/english/fucknovelpia.ts (all four checks pass), plus eslint and prettier --check on the file (both clean). No new tests, per the maintainers' request.

AI-authored change (original fix and review follow-up); no human review.