Fix non-recurring event update functionality by using actual CalDAV hrefs

This commit resolves the 404 error when dragging/updating non-recurring events.

The issue was in the regular event update handler where it was generating fake
event hrefs based on the UID (format: "{uid}.ics") instead of using the actual
href returned by the CalDAV server during event fetching.

Changes:
- Use event.href from the stored event data instead of generating fake hrefs
- Add comprehensive debug logging to track event updates
- Fallback to generated href only when the stored href is missing

Debug logging added:
- "🔍 Found event {uid} with href: {href}" - shows which href is being used
- "📝 Updating event {uid} at calendar_path: {path}, event_href: {href}" - tracks update parameters
- "✅ Successfully updated event {uid}" - confirms successful CalDAV updates

This matches the fix previously applied to the deletion functionality and ensures
that CalDAV PUT requests target the correct resource URLs.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/src/handlers/events.rs

@@ -533,8 +533,9 @@ pub async fn update_event(
             Ok(events) => {
                 for event in events {
                     if event.uid == request.uid {
-                        // Generate event href from UID
+                        // Use the actual href from the event, or generate one if missing
-                        let event_href = format!("{}.ics", event.uid);
+                        let event_href = event.href.clone().unwrap_or_else(|| format!("{}.ics", event.uid));
+                        println!("🔍 Found event {} with href: {}", event.uid, event_href);
                         found_event = Some((event, calendar_path.clone(), event_href));
                         break;
                     }
@@ -590,9 +591,12 @@ pub async fn update_event(
     event.priority = request.priority;
 
     // Update the event on the CalDAV server
+    println!("📝 Updating event {} at calendar_path: {}, event_href: {}", event.uid, calendar_path, event_href);
     client.update_event(&calendar_path, &event, &event_href)
         .await
         .map_err(|e| ApiError::Internal(format!("Failed to update event: {}", e)))?;
+    
+    println!("✅ Successfully updated event {}", event.uid);
 
     Ok(Json(UpdateEventResponse {
         success: true,