fix(payments): compare global coupon redemptions against max_redemptions not max_redemptions_per_user

validate_coupon was checking:
  total_redemptions (= redemptions_used, global count) >= max_per_user (per-user limit)

which means a coupon with max_redemptions=1000 and max_redemptions_per_user=1
would be marked exhausted after the first person ever used it.

Fix: also SELECT max_redemptions and compare global count against it.
The per-user check on line 192 (user_redemptions >= max_per_user) was
already correct and is unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/payments/src/ai_credits.rs

@@ -151,13 +151,14 @@ async fn validate_coupon(
     original_price: i32,
 ) -> Result<Option<(i32, String, i32)>, String> {
     // Get coupon details
-    let coupon: Option<(Uuid, String, i32, Option<i32>, i32, i32, Vec<Uuid>)> = sqlx::query_as(
+    let coupon: Option<(Uuid, String, i32, Option<i32>, i32, i32, Vec<Uuid>, i32)> = sqlx::query_as(
         r#"
         SELECT
             id, discount_type,
             CAST(discount_value * 100 AS INTEGER) as discount_value,
             CAST(max_discount_amount * 100 AS INTEGER) as max_discount_amount,
-            max_redemptions_per_user, redemptions_used, applicable_package_ids
+            max_redemptions_per_user, redemptions_used, applicable_package_ids,
+            max_redemptions
         FROM ai_coupons
         WHERE code = $1
           AND is_active = TRUE
@@ -170,12 +171,12 @@ async fn validate_coupon(
     .await
     .map_err(|e| format!("DB error: {e}"))?;
 
-    let Some((coupon_id, discount_type, discount_value, max_discount_amount, max_per_user, total_redemptions, applicable_packages)) = coupon else {
+    let Some((coupon_id, discount_type, discount_value, max_discount_amount, max_per_user, total_redemptions, applicable_packages, global_limit)) = coupon else {
         return Ok(None); // Coupon not found or invalid
     };
 
-    // Check if coupon has remaining redemptions
-    if total_redemptions >= max_per_user {
+    // Check if coupon has remaining global redemptions
+    if total_redemptions >= global_limit {
         return Err("Coupon redemption limit reached".to_string());
     }