feat: fetch sitemap dynamically

[Alessandro100]

Summary:

closes #19

This PR consumes the dynamically generated sitemap that is created from MobilityData/mobility-feed-api#1802. It has a cache life of 1 hour.

Expected behavior:

When a user (mainly bots) requests the sitemap by going on mobilitydatabase/sitemap.xml nextjs will check if it exists in cache, if yes, it will return it, if not, it will fetch the sitemap from the specified GCP bucket

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with yarn test to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

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Project	Deployment	Actions	Updated (UTC)
mobilitydatabase-web	Ready	Preview	Aug 20, 2026 12:09pm

[github-actions]

*Lighthouse ran on https://mobilitydatabase-66v0ah2je-mobility-data.vercel.app/ * (Desktop)
⚡️ HTML Report Lighthouse report for the changes in this PR:

Performance	Accessibility	Best Practices	SEO
🟢 92	🟢 94	🟢 96	🟢 100

*Lighthouse ran on https://mobilitydatabase-66v0ah2je-mobility-data.vercel.app/feeds * (Desktop)
⚡️ HTML Report Lighthouse report for the changes in this PR:

Performance	Accessibility	Best Practices	SEO
🟠 78	🟠 87	🟢 96	🟢 100

*Lighthouse ran on https://mobilitydatabase-66v0ah2je-mobility-data.vercel.app/feeds/gtfs/mdb-2126 * (Desktop)
⚡️ HTML Report Lighthouse report for the changes in this PR:

Performance	Accessibility	Best Practices	SEO
🔴 29	🟢 94	🟢 96	🟢 100

*Lighthouse ran on https://mobilitydatabase-66v0ah2je-mobility-data.vercel.app/feeds/gtfs_rt/mdb-2585 * (Desktop)
⚡️ HTML Report Lighthouse report for the changes in this PR:

Performance	Accessibility	Best Practices	SEO
🟠 86	🟠 84	🟢 96	🟢 100

*Lighthouse ran on https://mobilitydatabase-66v0ah2je-mobility-data.vercel.app/feeds/gbfs/gbfs-flamingo_porirua * (Desktop)
⚡️ HTML Report Lighthouse report for the changes in this PR:

Performance	Accessibility	Best Practices	SEO
🟢 95	🟢 94	🟢 96	🟢 100