build(deps): update next.js to 15.5.9 for security patch

[CruzMolina]

Address DoS vulnerability GHSA-5j59-xgg2-r9c4 in Next.js Server Components (incomplete fix follow-up for CVE-2025-55184).

• Bump next from 15.5.8 to 15.5.9
• Bump eslint-config-next from 15.5.8 to 15.5.9

---

Note

Updates Next.js and eslint-config-next to 15.5.9 and adds a routes types reference to next-env.d.ts.

• Dependencies:
  • Bump next to 15.5.9.
  • Bump eslint-config-next to 15.5.9.
• TypeScript:
  • Add /// <reference path="./.next/types/routes.d.ts" /> in next-env.d.ts.

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