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.

beancount_format.py

@@ -889,7 +889,7 @@ def format_revenue_entry(
 
 def format_income_entry(
     user_id: str,
-    payment_account: str,
+    user_account: str,
     revenue_account: str,
     amount_sats: int,
     description: str,
@@ -906,7 +906,8 @@ def format_income_entry(
     fiat-first price notation (@@ SATS) for BQL queryability, unique link
     (^inc-{entry_id}) for tracking through the approve/reject flow.
 
-    Postings: DR payment_account (asset receives funds), CR revenue_account.
+    Postings: DR user_account (Assets:Receivable:User-{id} — user owes
+    the entity until they hand the cash over), CR revenue_account.
     """
     if not fiat_currency or not fiat_amount or fiat_amount <= 0:
         raise ValueError("fiat_currency and a positive fiat_amount are required for income entries")
@@ -921,7 +922,7 @@ def format_income_entry(
 
     postings = [
         {
-            "account": payment_account,
+            "account": user_account,
             "amount": f"{fiat_amount_abs:.2f} {fiat_currency} @@ {sats_abs} SATS",
         },
         {