feat(billing): add billing_mode to providers (postpaid support) & UI/migration

src/logging/mod.rs

@@ -100,13 +100,18 @@ impl RequestLogger {
         .execute(&mut *tx)
         .await?;
 
-        // 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)
-                .execute(&mut *tx)
-                .await?;
+        // Deduct from provider balance if successful.
+        // Providers configured with billing_mode = 'postpaid' will not have their
+        // credit_balance decremented. Use a conditional UPDATE so we don't need
+        // a prior SELECT and avoid extra round-trips.
+        if log.cost > 0.0 {
+            sqlx::query(
+                "UPDATE provider_configs SET credit_balance = credit_balance - ? WHERE id = ? AND (billing_mode IS NULL OR billing_mode != 'postpaid')",
+            )
+            .bind(log.cost)
+            .bind(&log.provider)
+            .execute(&mut *tx)
+            .await?;
         }
 
         tx.commit().await?;