Add cursor-based pagination for large tip history endpoints

[nanaf6203-bit]

Area: Database Performance
Priority: P2 — Medium

Description

Offset pagination becomes slow at high pages because the database must scan and skip all preceding rows. Cursor-based pagination is O(log n) and stable for new data. Replace offset with cursors on hot endpoints.

Acceptance Criteria

• Add optional cursor query param to GET /tips/my/received, GET /tips/my/sent, GET /tips/wallet/:address
• Cursor format: base64-url-encoded JSON {createdAt,id} tuple
• Response includes nextCursor and previousCursor (omitted when no more data)
• Backward-compatible: offset pagination (page/limit) still works
• Implement using WHERE (createdAt, id) < (?, ?) for forward cursors
• Tests verifying no duplicate/missed rows across cursor pages when new data appears
• Document cursor API in README under API > Tips > Pagination

Technical Notes

• Use Buffer.from(JSON.stringify({t,i})).toString('base64url') for encoding
• Validate cursor shape; bad cursor → 400
• Limit + 1 query trick to detect nextCursor presence

[Jayydy]

Hello, can i be assigned to this issue?

[grantfox-oss]

🦊 GrantFox — @Jayydy has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #67)
2. Your PR will be reviewed by the StellarTips maintainers

Good luck! Track your progress on GrantFox.