Surface user credit balance in GET /balance per libra-#41

Extends get_user_balance_bql and get_all_user_balances_bql to fold
Liabilities:Credit:User-X into the same query as Payable and Receivable.
Credit is the overpay-absorbing liability that libra owes the user going
forward — it carries the same sign as Payable, so the existing fiat
aggregation subtracts it from net obligation without further changes.

Adds UserBalance.account_balances to surface the BQL per-account
breakdown so libra extension UI and webapp can render Payable /
Receivable / Credit as distinct line items. The legacy `accounts` field
stays empty for back-compat with anything reading the older shape.

Prepares for libra-#33 / libra-#41: settlement netting (#14 task) will
write the overflow leg to credit; this changeset makes sure that, the
moment credit exists, the displayed net everywhere already reflects it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

views_api.py

@@ -1609,7 +1609,8 @@ async def api_get_my_balance(
     return UserBalance(
         user_id=wallet.wallet.user,
         balance=balance_data["balance"],
-        accounts=[],  # Could populate from balance_data["accounts"] if needed
+        accounts=[],
+        account_balances=balance_data.get("accounts", []),
         fiat_balances=balance_data["fiat_balances"],
         total_expenses_sats=totals["total_expenses_sats"],
         total_expenses_fiat=totals["total_expenses_fiat"],