refactor(api): standardize cursor-based pagination across list endpoi…

[Killerjunior]

refactor(api): standardize cursor-based pagination across list endpoints (#344)

Summary

Standardizes cursor-based pagination across every list endpoint in the backend with uniform request parameters (cursor, limit default 20, max 100) and canonical response shapes ({ data: [...], pagination: { nextCursor, hasMore, total } }).

Type of change

• Bug fix
• New feature
• Documentation update
• Refactor / chore
• Smart contract change

Related issue

Closes #344

Changes

• Pagination Utility & Formatter (backend/src/utils/paginate.js): Added formatPaginatedResponse(data, cursor, total, extra) to standardize the payload shape across in-memory and database-backed list queries while preserving RFC 5988 Link (rel="next") and X-Total-Count headers.
• Pagination Middleware (backend/src/middleware/pagination.js & backend/src/server.js): Applied paginationMiddleware globally to sanitize limit (default 20, capped at 100) and parse opaque base64url cursor tokens across all routes.
• Account & Payment Endpoints (accounts.js, payments.js, paymentController.js): Added GET /api/v1/accounts/:publicKey/payments and updated GET /api/v1/payments/:publicKey to return standard paginated envelopes using Horizon paging tokens as cursors.
• Tips & Events Endpoints (tips.js, tipsController.js, events.js, eventController.js): Standardized GET /api/v1/tips/received/:creatorPublicKey, GET /api/v1/tips/sent/:senderPublicKey, GET /api/v1/tips/:creatorPublicKey, GET /api/v1/events/:publicKey, and GET /api/v1/events/:publicKey/:eventType.
• Webhooks, Turrets & Scheduled Transactions (webhooks.js, turrets.js, turretsController.js, scheduledTransactions.js): Standardized GET /api/v1/webhooks/:publicKey, GET /api/v1/webhooks/:publicKey/events, GET /api/v1/turrets, GET /api/v1/turrets/:id/history, GET /api/v1/scheduled-transactions/:publicKey, and GET /api/v1/scheduled-transactions/:publicKey/pending.
• API Documentation (docs/api.md): Added a dedicated "Pagination" section detailing parameters, response structures, header definitions, and all supported endpoints.
• Test Suite (backend/__tests__/pagination.test.js): Added unit tests for formatPaginatedResponse and integration tests validating cursor pagination across all list endpoints.

Testing

• Tested locally on Testnet
• Added/updated unit tests
• Manually tested UI flow

Screenshots (if UI change)

N/A (Backend API standardization)

Checklist

• My code follows the project style
• I've updated docs if needed
• No console errors or warnings
• I've rebased on latest main

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