Record income receipts as a user receivable, not an entity asset

When a user submits income, the money is physically in *their* pocket,
not the entity's cash drawer. The original income endpoint posted DR
on a configurable payment-method asset account (Cash/Bank/Lightning),
which implicitly assumed the entity already had the funds.

Mirror the expense flow instead: DR Assets:Receivable:User-{id[:8]}
(via get_or_create_user_account), CR the revenue account. The user
now owes the entity until they hand the cash over via the existing
/settle-receivable workflow. With this, the per-user Outstanding
Balances card correctly nets expenses (entity owes user, -liability)
against income receipts (user owes entity, +receivable).

Drops payment_method_account from IncomeEntry — no longer needed.

models.py

@@ -128,12 +128,18 @@ class RevenueEntry(BaseModel):
 
 
 class IncomeEntry(BaseModel):
-    """Helper model for user-facing income/revenue submission (pending approval)"""
+    """Helper model for user-facing income/revenue submission (pending approval).
+
+    The user records that they personally received money on the entity's
+    behalf — so the postings are DR Assets:Receivable:User-{id} / CR
+    revenue_account. The user now owes the entity until they settle via
+    the existing /settle-receivable flow. Symmetric with ExpenseEntry,
+    which credits Liabilities:Payable:User-{id} (entity owes user).
+    """
 
     description: str
     amount: Decimal  # Fiat amount in the specified currency
     revenue_account: str  # Income/Revenue account name or ID
-    payment_method_account: str  # Asset account receiving the funds (Cash, Bank, Lightning)
     currency: str  # Required: fiat currency code (EUR, USD, etc.)
     reference: Optional[str] = None
     entry_date: Optional[datetime] = None