Adds settle receivable functionality

Implements a "Settle Receivable" feature for super users to record manual payments from users who owe money.

Introduces a dialog for inputting payment details (amount, method, description, reference), triggers an API call to record the transaction, and updates user balances and transaction history.

This is for non-lightning payments like cash, bank transfers, or checks.

models.py

@@ -181,6 +181,16 @@ class RecordPayment(BaseModel):
     payment_hash: str
 
 
+class SettleReceivable(BaseModel):
+    """Manually settle a receivable (user pays castle in person)"""
+
+    user_id: str
+    amount: int  # Amount in satoshis
+    payment_method: str  # "cash", "bank_transfer", "lightning", "other"
+    description: str  # Description of the payment
+    reference: Optional[str] = None  # Optional reference (receipt number, transaction ID, etc.)
+
+
 class AssertionStatus(str, Enum):
     """Status of a balance assertion"""
     PENDING = "pending"    # Not yet checked