fix: cursor pagination for vault transactions endpoint

[Fang0067]

Overview

This PR fixes offset-based pagination on GET /vaults/:id/transactions, which could skip or duplicate rows when new transactions are inserted between page fetches. It adds cursor-based pagination using ?cursor=<timestamp>_<id>, keeps ?page for backward compatibility (deprecated), and exposes a nextCursor field in the response.

Related Issue

Changes

📄 Cursor Pagination

• [MODIFY] backend/src/pagination.ts

  • Added cursor encode/decode helpers (<timestamp>_<id>).
  • Added cursor-based where clause using (timestamp, id) < cursor with orderBy: [{ timestamp: 'desc' }, { id: 'desc' }].
  • Kept existing offset (?page) logic intact for backward compatibility and marked it deprecated.

• [MODIFY] backend/src/routes/transactions.ts

  • Accept ?cursor on GET /vaults/:id/transactions; prefer cursor over ?page when both are present.
  • Return nextCursor in the response payload.

• [MODIFY] backend/openapi.json

  • Documented cursor query parameter as the preferred pagination method.
  • Marked page as deprecated.
  • Added nextCursor to the response schema.

• [MODIFY] backend/schema-snapshots/get-_api_v1_transactions.json

  • Updated snapshot to include the nextCursor field.

🧪 Tests

• [ADD] backend/src/__tests__/pagination.test.ts

  • Unit coverage for cursor encode/decode and cursor where/orderBy construction.

• [MODIFY] backend/src/__tests__/transactions.test.ts

  • Inserts 3 txs, fetches page 1 (limit 2), inserts a new tx, then fetches page 2 via cursor and asserts no skipped or duplicated rows.

Verification Results

npm test -- backend/src/__tests__/pagination.test.ts backend/src/__tests__/transactions.test.ts
✅ pagination.test.ts passed
✅ transactions.test.ts passed (cursor pagination: no skip/duplicate after insert)

Acceptance Criteria	Status
Add cursor pagination: support ?cursor=<timestamp>_<id> with where: { (timestamp, id) < cursor } and orderBy: [{timestamp: 'desc'}, {id: 'desc'}]	✅ Implemented in pagination.ts and wired into the transactions route
Keep ?page for backward compat but deprecate and document cursor as preferred	✅ ?page still works; marked deprecated in openapi.json; cursor documented as preferred
Test: insert 3 txs, fetch page 1 (limit 2), insert new tx, fetch page 2 via cursor and assert no skip/duplicate	✅ Covered in transactions.test.ts
Snapshot includes nextCursor field	✅ Updated get-_api_v1_transactions.json and response schema

Closes #1444

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