feat(economy): add generic purchase_item (atomic debit + mint)

The FIFA17 economy audit proved the transfer market is synthetic-seller:
buy-now mints a new owned item and debits the buyer; no real counterparty,
no sale-credit/expiry/fee. The generic primitive that models this is an
atomic debit + inventory add (purchase_item), NOT a two-party
transfer_item_with_payment (which would be unused). Fail-closed: an
unaffordable purchase debits nothing and mints nothing. 2 unit tests.

src/services/economy.rs

@@ -212,6 +212,25 @@ pub async fn purchase_entitlement(
     })
 }
 
+/// Debit `cost` and mint one owned item, atomically. Fail-closed: if the club
+/// cannot afford `cost`, nothing is debited and no item is added. This is the
+/// "buy a specific item" primitive (a debit paired with an inventory add), for
+/// synthetic-seller markets where the purchased item is minted rather than
+/// transferred from another owner. Returns the post-debit balance.
+pub async fn purchase_item(
+    pool: &Pool,
+    club_id: &str,
+    cost: i64,
+    item_id: &str,
+    card_id: &str,
+) -> AppResult<i64> {
+    let mut tx = pool.begin().await?;
+    let balance = debit(&mut tx, club_id, cost).await?;
+    add_item(&mut tx, club_id, item_id, card_id).await?;
+    tx.commit().await?;
+    Ok(balance)
+}
+
 /// Consume an entitlement once and add its granted items, atomically. If any
 /// item insert fails (e.g. a colliding instance id) the whole redemption rolls
 /// back — the entitlement stays unconsumed and no items are persisted.
@@ -452,4 +471,27 @@ mod tests {
         assert_eq!(grant_reward(&pool, "club", 500).await.unwrap(), 1500);
         assert_eq!(balance(&pool, "club").await.unwrap(), 1500);
     }
+
+    #[tokio::test]
+    async fn purchase_item_debits_and_mints() {
+        let pool = fixture().await;
+        let bal = purchase_item(&pool, "club", 400, "item-new", "def-new")
+            .await
+            .unwrap();
+        assert_eq!(bal, 600);
+        assert_eq!(balance(&pool, "club").await.unwrap(), 600);
+        assert_eq!(item_count(&pool, "item-new").await, 1);
+    }
+
+    #[tokio::test]
+    async fn purchase_item_insufficient_funds_rolls_back() {
+        let pool = fixture().await;
+        let err = purchase_item(&pool, "club", 9000, "item-new", "def-new")
+            .await
+            .unwrap_err();
+        assert!(matches!(err, AppError::BadRequest(_)));
+        // Nothing debited, no item minted.
+        assert_eq!(balance(&pool, "club").await.unwrap(), 1000);
+        assert_eq!(item_count(&pool, "item-new").await, 0);
+    }
 }