feat(prices): add management command to fix missing currency

[Kinjal0007]

Problem

Some Price entries have null currency, which causes issues for clients like Smoothie that assume currency is always present.

Fix

This PR adds a non-breaking management command fix_missing_price_currency:

• Reports records with missing currency
• With --write, fills currency using:
  • proof.currency if available
  • fallback from location.osm_address_country_code

Notes

• Does not change model or validation behavior
• Keeps existing API contract
• Provides a safe way to clean legacy records

Usage

make cli args="fix_missing_price_currency"
make cli args="fix_missing_price_currency --write"