fix: user notification/request counts

crates/core/src/model/mail.rs

@@ -17,11 +17,19 @@ pub struct Letter {
     /// This field can be updated by anyone in the letter's `receivers` field.
     /// Other fields in the letter can only be updated by the letter's `owner`.
     pub read_by: Vec<usize>,
+    /// The ID of the letter this letter is replying to.
+    pub replying_to: usize,
 }
 
 impl Letter {
     /// Create a new [`Letter`].
-    pub fn new(owner: usize, receivers: Vec<usize>, subject: String, content: String) -> Self {
+    pub fn new(
+        owner: usize,
+        receivers: Vec<usize>,
+        subject: String,
+        content: String,
+        replying_to: usize,
+    ) -> Self {
         Self {
             id: Snowflake::new().to_string().parse::<usize>().unwrap(),
             created: unix_epoch_timestamp(),
@@ -30,6 +38,7 @@ impl Letter {
             subject,
             content,
             read_by: Vec::new(),
+            replying_to,
         }
     }
 }