Add CSV/JSON export for expenses and bills

[sofia-willow]

Problem

The README mentions CSV/Excel export as a premium feature, but there's no implementation yet. Users need a way to export their financial data for tax prep, spreadsheet analysis, or migration.

Proposed Solution

Add export endpoints:

• GET /expenses/export?format=csv — Export expenses as CSV
• GET /expenses/export?format=json — Export expenses as JSON array
• GET /bills/export?format=csv — Export bills as CSV

Query Parameters

• format: csv or json (default: csv)
• from / to: Date range filter
• category_id: Filter by category

CSV Format

date,amount,currency,category,type,notes
2026-03-15,42.50,USD,Food,EXPENSE,Lunch with team

Implementation Notes

• Uses stdlib csv module (no new dependencies)
• Proper Content-Disposition header for file download
• Respects existing auth (JWT required)
• Integrates with existing expense query filters

Acceptance Criteria

• Export endpoints return valid CSV/JSON
• Date range filtering works
• Auth-protected
• Tests included
• No new dependencies

I can submit a PR for this if there's interest.