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>

fava_client.py

@@ -820,10 +820,15 @@ class FavaClient:
         # GROUP BY currency prevents mixing EUR and SATS face values in sum(number).
         # sum(weight) gives SATS for both EUR @@ SATS entries and plain SATS entries.
         # sum(number) on EUR rows gives the fiat amount; on SATS rows gives sats paid.
+        # Credit is the overpay-absorbing liability per libra-#41 — it lives
+        # on the same per-user namespace as Payable and contributes to the
+        # user's net obligation with the same sign as Payable (negative on
+        # Liabilities means libra owes user). Folding it into the same query
+        # means the displayed net always already accounts for credit.
         query = f"""
             SELECT account, currency, sum(number), sum(weight)
             WHERE account ~ ':User-{user_id_prefix}'
-            AND (account ~ 'Payable' OR account ~ 'Receivable')
+            AND (account ~ 'Payable' OR account ~ 'Receivable' OR account ~ 'Credit')
             AND flag = '*'
             GROUP BY account, currency
         """
@@ -970,10 +975,11 @@ class FavaClient:
         """
         from decimal import Decimal
 
-        # GROUP BY currency prevents mixing EUR and SATS face values in sum(number)
+        # GROUP BY currency prevents mixing EUR and SATS face values in sum(number).
+        # Credit per libra-#41 — see get_user_balance_bql for the rationale.
         query = """
             SELECT account, currency, sum(number), sum(weight)
-            WHERE (account ~ 'Payable:User-' OR account ~ 'Receivable:User-')
+            WHERE (account ~ 'Payable:User-' OR account ~ 'Receivable:User-' OR account ~ 'Credit:User-')
             AND flag = '*'
             GROUP BY account, currency
         """