feat: Add JSON serialization/deserialization to Event struct

- Added `to_json` method to serialize Event to JSON string.
- Added `from_json` method to deserialize Event from JSON string.
- This allows for easier data exchange and persistence.

heromodels/src/models/calendar/calendar.rs

@@ -101,6 +101,16 @@ pub struct Event {
 }
 
 impl Event {
+    /// Converts the event to a JSON string
+    pub fn to_json(&self) -> Result<String, serde_json::Error> {
+        serde_json::to_string(self)
+    }
+
+    /// Creates an event from a JSON string
+    pub fn from_json(json: &str) -> Result<Self, serde_json::Error> {
+        serde_json::from_str(json)
+    }
+
     /// Creates a new event with auto-generated ID
     pub fn new(title: impl ToString, start_time: DateTime<Utc>, end_time: DateTime<Utc>) -> Self {
         Self {