feat: gemini postpaid billing (skip credit_balance deduction)

src/logging/mod.rs

@@ -100,8 +100,8 @@ impl RequestLogger {
         .execute(&mut *tx)
         .await?;
 
-        // Deduct from provider balance if successful
-        if log.cost > 0.0 {
+        // Deduct from provider balance if successful (skip postpaid like Gemini)
+        if log.cost > 0.0 && log.provider != "gemini" {
             sqlx::query("UPDATE provider_configs SET credit_balance = credit_balance - ? WHERE id = ?")
                 .bind(log.cost)
                 .bind(&log.provider)