[ MostroP2P/mostro ] feat: add development fund configuration infrastructure (#555) * feat: add development fund configuration infrastructure Introduces configurable development fee to support sustainable Mostro development. Operators can set dev_fee_percentage (10-100% of Mostro fee) to automatically contribute to development fund on each successful order. - Add dev_fee_percentage config with startup validation (default: 30%) - Database migration: dev_fee, dev_fee_paid, dev_fee_payment_hash columns