feat: Add /onchain command to settle buyer payment on-chain (swap Lightning → on-chain)

[Matobi98]

Summary

Allow the buyer to optionally request their payout in on-chain Bitcoin instead of Lightning, by issuing a /onchain command before the seller releases funds. The bot would perform a submarine swap (Lightning → on-chain) and charge an additional fee to cover swap costs.

Proposed UX Flow

1. A buy or sell order reaches the WAITING_BUYER_INVOICE state (bot is waiting for the buyer to supply a Lightning invoice).
2. Instead of sending a Lightning invoice, the buyer sends /onchain <bitcoin_address>.
3. The bot validates the address (mainnet bech32 / p2pkh / p2sh).
4. The bot replies with a fee breakdown and asks for confirmation:

⛓️ On-chain settlement requested

Amount:           100,000 sats
Swap fee:           1,500 sats (1.5%)
Network fee:          500 sats (estimated)
────────────────────────────
You will receive:  ~98,000 sats

Address: bc1q...xyz

Confirm? [Yes] [No]

5. On confirmation, the order is flagged with onchain_payout: true and the address is stored.
6. The rest of the flow is unchanged: seller pays the hold invoice → both parties confirm fiat → seller releases.
7. At release, instead of paying a Lightning invoice, the bot performs a submarine swap (e.g. via Loop Out, Boltz, or a configured swap provider) and sends on-chain.
8. The swap fee is recorded on the order and shown in the final receipt.

Data Model Changes

New fields added to the Order schema:

• onchain_payout — boolean, flags the order for on-chain settlement
• onchain_address — the destination Bitcoin address provided by the buyer
• onchain_tx_id — the txid after the swap is broadcast
• onchain_fee_sats — the actual swap fee charged, in satoshis

Fee Structure

Two new configurable environment variables:

Variable	Description	Default
ONCHAIN_SWAP_FEE_PERCENT	Percentage of the order amount charged for the swap	0.015 (1.5%)
ONCHAIN_MIN_AMOUNT_SATS	Minimum order size to allow on-chain settlement	10000

The existing MAX_FEE and community FEE_PERCENT are applied first. The swap fee is additive and shown separately in the breakdown.

New Command

/onchain <bitcoin_address>

• Only available to the buyer of the order
• Only valid while the order is in WAITING_BUYER_INVOICE status
• Replaces the need to provide a Lightning invoice for that order
• Disabled if the order amount is below ONCHAIN_MIN_AMOUNT_SATS