[Backend] Streaming Audit Export `GET /transactions/export`

[Olowodarey]

Background & Context: Required standard feature supporting users auditing personal finances downloading a spreadsheet locally.
Description: Dynamically orchestrate DB logic piping transactions via Node Streams formatting raw Comma-Separated Values explicitly pushed as a user download request.
Acceptance Criteria:

• Isolate same query limits processing inside a stream wrapper extracting data incrementally to maintain RAM headroom on massive multi-year dataset conversions.
• Apply library fast-csv or json2csv executing transformations cleanly.
• Set critical express HTTP headers: res.setHeader('Content-Type', 'text/csv') and res.setHeader('Content-Disposition', 'attachment; filename="nestera_history.csv"').
  Target Files: backend/src/modules/transactions/transactions.service.ts.