Remove v2 API endpoints and fix warnings

- Remove all v2 API routes (/api/v2/calendar/events/*)
- Delete models_v2.rs file and associated types
- Remove create_event_v2, update_event_v2, delete_event_v2 handlers
- Remove unused occurrence_date and exception_dates from UpdateEventRequest
- Remove unused ConfigError variant from CalDAVError
- Simplify backend to single unified v1 API using VEvent structures

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

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

backend/src/calendar.rs

@@ -1060,9 +1060,6 @@ pub enum CalDAVError {
     
     #[error("Failed to parse calendar data: {0}")]
     ParseError(String),
-    
-    #[error("Configuration error: {0}")]
-    ConfigError(String),
 }
 
 #[cfg(test)]