Replace fragile price change detection in Purchase.save() with a
lazy dirty flag approach. A pre_save/post_save signal pair detects
price/currency changes without extra DB queries, and convert_prices()
uses the flag to determine which purchases need conversion.
- Add needs_price_update BooleanField with db_index
- Add pre_save signal to store old price/currency values
- Add post_save signal to set needs_price_update=True when price/currency changes
- Simplify Purchase.save() to remove DB reload + comparison logic
- Remove price_or_currency_differ_from() method
- Update convert_prices() to filter on needs_price_update flag
- Extract _get_exchange_rate() and _save_converted_price() helpers
- Add tests for the new behavior
