✨ feat(retention): introduce message age enum

- add MessageAge enum to represent retention periods in days, weeks, months, or years
- implement Display trait for MessageAge to format retention periods
- implement label function to generate retention label

src/retention/message_age.rs

@@ -0,0 +1,36 @@
+use std::fmt::Display;
+
+/// Message age
+#[derive(Debug)]
+pub enum MessageAge {
+    /// Number of days to retain the message
+    Days(usize),
+    /// Number of weeks to retain the message
+    Weeks(usize),
+    /// Number of months to retain the message
+    Months(usize),
+    /// Number of years to retain the message
+    Years(usize),
+}
+
+impl Display for MessageAge {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            MessageAge::Days(v) => write!(f, "d:{v}"),
+            MessageAge::Weeks(v) => write!(f, "w:{v}"),
+            MessageAge::Months(v) => write!(f, "m:{v}"),
+            MessageAge::Years(v) => write!(f, "y:{v}"),
+        }
+    }
+}
+
+impl MessageAge {
+    pub(crate) fn label(&self) -> String {
+        match self {
+            MessageAge::Days(v) => format!("retention/{v}-days"),
+            MessageAge::Weeks(v) => format!("retention/:{v}-weeks"),
+            MessageAge::Months(v) => format!("retention/:{v}-months"),
+            MessageAge::Years(v) => format!("retention/:{v}-years"),
+        }
+    }
+}