feat: add cursor pagination to list_programs and list_payouts (Closes #19)

[ZacLou]

Closes #19

Changes

Adds cursor-based pagination to list_programs and list_payouts endpoints:

• Query parameters: ?cursor=<ISO 8601 timestamp>&limit=<n> (default limit: 20, max: 100)
• Response format: { "items": [...], "next_cursor": "<timestamp>", "has_more": bool }
• Pagination logic: Keyset pagination on created_at, ordered DESC
  • When cursor is provided, filters WHERE created_at < cursor
  • next_cursor is the created_at of the last item in the page
  • has_more: true when a full page was returned (meaning more items exist)

Types added

• PaginationParams — query deserialization struct
• ListResponse<T> — generic paginated response wrapper

Endpoints updated

• GET /api/v1/programs?limit=20&cursor=2026-01-01T00:00:00Z
• GET /api/v1/programs/:id/payouts?limit=20&cursor=...

Backward compatible: omitting query params returns first page of 20 items (same as before).