add: coin purchases + donator badge

crates/core/src/model/auth.rs

@@ -95,6 +95,12 @@ pub struct User {
     /// The number of coins the user has.
     #[serde(default)]
     pub coins: i32,
+    /// The IDs of Stripe checkout sessions that this user has successfully completed.
+    ///
+    /// This should be checked BEFORE applying purchases to ensure that the user hasn't
+    /// already applied this purchase.
+    #[serde(default)]
+    pub checkouts: Vec<String>,
 }
 
 pub type UserConnections =
@@ -407,6 +413,7 @@ impl User {
             is_deactivated: false,
             ban_expire: 0,
             coins: 0,
+            checkouts: Vec::new(),
         }
     }