feat(seo): include active trade listings in the sitemap

[Hey-Yetunde]

Closes #381

Summary

frontend/app/sitemap.ts now fetches the active trade listings and appends their /trades/<id> URLs, so search engines can discover and index individual trade pages.

Changes (frontend/app/sitemap.ts)

• sitemap() is now an async route handler that returns the static PUBLIC_ROUTES plus dynamic trade entries.
• Trades are fetched from the public listing API (${NEXT_PUBLIC_API_URL ?? http://localhost:3001}/api/v1/trades?page=&limit=), the same endpoint the home page uses. Up to 5 pages × 100 are pulled for a bounded sitemap.
• Only status === "Active" trades are included.
• Each entry uses trade.updated_at as lastModified (/trades/<id>, changeFrequency: "daily", priority: 0.6).
• ISR: export const revalidate = 900 (15 minutes) and the fetch carries next: { revalidate: 900 }, so the sitemap regenerates on each revalidation cycle.
• Failure-safe: if the API is unreachable/returns a non-OK status, the sitemap degrades to the static routes instead of breaking the build.

Tests (frontend/app/lib/seo.test.ts)

• Existing sitemap tests updated for the async handler.
• New sitemap trade listings (Issue #381):
  • includes active trade URLs with lastmod from updated_at — asserts a dynamic /trades/trade-1 URL is present, that a Completed trade is excluded, and that lastModified equals the trade's updated_at.
  • degrades to static routes when the listing API fails — asserts the static / remains and no /trades/ entry is emitted.

Verification

• npx jest app/lib/seo.test.ts → 16 passed.
• npx tsc --noEmit → no errors in the changed files (3 pre-existing errors elsewhere).

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