fix(seo): resolve 300+ compare page 404s via dynamic SSR + 3-card support

[phucbm]

Problem

300+ /so-sanh/X-vs-Y URLs reported as 404 in Google Search Console this week. Root causes:

1. Static export on Cloudflare Pages only serves API-curated pairs — unknown pairs 404
2. Existing SoSanh404Redirect is client-side JS only — Google sees HTTP 404, not a redirect
3. Root-level /X-vs-Y URLs (missing /so-sanh/ prefix) had no redirect handler
4. 3-card comparison URLs (/so-sanh/A-vs-B-vs-C) were broken due to indexOf('-vs-') only finding first match

Solution

Migrated from Cloudflare Pages static export to Vercel SSR.

Changes (merged in ef56792, 1429b33)

app/(marketing)/so-sanh/[pair]/page.tsx

• dynamicParams = true — unknown pairs render via SSR instead of 404
• revalidate = 3600 — ISR caching for dynamically rendered pairs
• URL parsing changed from indexOf('-vs-') to split('-vs-') — supports 2 or 3 card slugs
• Title renders all card names: cards.map(c => c.name).join(' vs ')

next.config.ts

• Added server-side 301 redirect: /:pair([^/]*-vs-[^/]*) → /so-sanh/:pair
• Handles root-level compare URLs, fires only on single-segment paths (no redirect loop)

.env.example

• Replaced real API key with placeholder your-api-key-here

Result

• Curated pairs: pre-built static, fast, indexed ✓
• Unknown 2-card pairs: SSR on first request, cached 1h, indexable by Google ✓
• 3-card pairs (/so-sanh/A-vs-B-vs-C): SSR, full 3-column comparison ✓
• Root-level /X-vs-Y: server-side 301 → /so-sanh/X-vs-Y ✓
• Invalid pairs (card not found): real 404 ✓

Follow-up

• Submit affected URLs to Google Search Console for recrawl
• Monitor Search Console for 404 count drop over next 2 weeks

[phucbm]

Test results — production verified ✓

URL	Before	After
/so-sanh/sacombank-uniq-vs-vib-lazcard	404	✅ 200
/so-sanh/vietcombank-visa-business-debit-vs-vietcombank-visa-corporate	404	✅ 200
/bvbank-jcb-sense-vs-vpbank-diamond-world (root-level, missing /so-sanh/ prefix)	404	✅ 308 → /so-sanh/bvbank-jcb-sense-vs-vpbank-diamond-world
/so-sanh/fake-card-vs-another-fake	404	✅ 404 (correct — cards don't exist)
/tin-tuc	200	✅ 200 (unaffected)

Submit affected URLs to Google Search Console for recrawl. 404 count should drop over next 1–2 weeks.