♻️ refactor(gateway): migrate expenses REST routes from file-based to PostgreSQL

Complete expense migration — both AI tools AND REST API now use DB:
- Rewrite expenses.ts routes (472→270 lines) to use ExpensesRepository
- Remove fs.readFile/writeFile, loadExpenseDb/saveExpenseDb
- Add getUserId() scoping to all endpoints
- Rewrite tests (413→200 lines) with ExpensesRepository mocks
- Add expenses card to DashboardPage + StatsPanel (orange, Receipt icon)
- Add expenses to SummaryData type

The ExpensesPage UI now works with PostgreSQL-backed data
instead of the old ~/.ownpilot/expenses.json file.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ersinkoc