feat(redirect): serve a friendly 410 page for expired links

[onamfc]

Summary

• Expired links now show a proper page. Opening a short link past its expires_at returns HTTP 410 Gone with a lightweight HTML page — clock icon, "This link has expired", one line of guidance, and a small "Powered by LinkForty" footer. Previously expired links returned the same bare 404 {"error":"Link not found"} as short codes that never existed. [1]
• Expired vs. never-existed is now distinguished. When the active-link lookup misses, a secondary query checks for a link with that short code whose expires_at has passed (both the template-slug and legacy URL forms). Since expired links keep their expires_at even after a housekeeping job deactivates them, the 410 page keeps serving indefinitely. Unknown short codes still 404. [1]
• Expiry is now enforced on Redis cache hits. A link cached shortly before expiring used to keep redirecting for up to the 5-minute cache TTL; the redirect now re-checks expires_at after a cache read, so links stop at the exact expiration moment. [1]
• The page carries noindex so expired links drop out of search engines, and it's served on whatever domain the short link uses. [1]
• New exported helpers isLinkExpired() (null/invalid timestamps fail open) and generateExpiredLinkHTML(), both unit-tested. [1] [2]

Test plan

• npm test — 145 tests pass, including new coverage for isLinkExpired (no expiry, future, past, exact-moment, unparseable) and the page content.
• npm run build clean.
• Manual: create a link with expiresAt in the near future, click it before expiry (redirect works), after expiry (410 page), and confirm an unknown short code still returns 404.

[codecov]

Codecov Report

❌ Patch coverage is 67.64706% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/routes/redirect.ts	67.64%	22 Missing ⚠️

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