Adds on-chain Bitcoin payment support

Adds support for on-chain Bitcoin payments by:
- Introducing a new `Assets:Bitcoin:OnChain` account.
- Updating the `SettleReceivable` and `PayUser` models to include `txid` for storing transaction IDs.
- Modifying the API endpoints to handle `btc_onchain` as a valid payment method and associate it with the new account.

This allows tracking on-chain Bitcoin transactions separately from Lightning Network payments.

migrations.py

@@ -332,3 +332,34 @@ async def m008_rename_lightning_account(db):
         WHERE name = 'Assets:Lightning:Balance'
         """
     )
+
+
+async def m009_add_onchain_bitcoin_account(db):
+    """
+    Add Assets:Bitcoin:OnChain account for on-chain Bitcoin transactions.
+    This allows tracking on-chain Bitcoin separately from Lightning Network payments.
+    """
+    import uuid
+
+    # Check if the account already exists
+    existing = await db.fetchone(
+        """
+        SELECT id FROM accounts
+        WHERE name = 'Assets:Bitcoin:OnChain'
+        """
+    )
+
+    if not existing:
+        # Create the on-chain Bitcoin asset account
+        await db.execute(
+            f"""
+            INSERT INTO accounts (id, name, account_type, description, created_at)
+            VALUES (:id, :name, :type, :description, {db.timestamp_now})
+            """,
+            {
+                "id": str(uuid.uuid4()),
+                "name": "Assets:Bitcoin:OnChain",
+                "type": "asset",
+                "description": "On-chain Bitcoin wallet"
+            }
+        )