Speed up QR create/update by cutting billing round-trips.

Create was making three sequential public HTTPS billing calls (~400ms each)
while holding a DB transaction open for image generation. Use loopback DNS
forcing, cache balances briefly, return balance from debit, regenerate images
only when style changes, and keep remote work outside the DB transaction.

.env.example

@@ -31,6 +31,8 @@ LADILL_SSO_CLIENT_ID=
 LADILL_SSO_CLIENT_SECRET=
 
 BILLING_API_URL=https://ladill.com/api/billing
+# Same-server: set to 127.0.0.1 to skip public hairpin latency on billing calls
+BILLING_API_FORCE_IP=127.0.0.1
 BILLING_API_KEY_QR=
 
 IDENTITY_API_URL=https://ladill.com/api