Add GET /api/exchange/price endpoint:
- Available to regular users (BOOK_APPOINTMENT permission)
- Returns current BTC/EUR price with admin premium applied
- Uses cached price from PriceHistory if not stale
- Fetches fresh price from Bitfinex if needed
- Returns is_stale flag when price is older than 5 minutes
- Includes exchange configuration (min/max EUR, increment)
- Handles fetch failures gracefully (returns stale price with error)
