Implement RFC 5545-compliant calendar system with v2 API

This major refactor eliminates manual string parsing throughout the
codebase and introduces proper RFC 5545 iCalendar specification
compliance with significant code simplification benefits.

## Backend Improvements
- Add complete RFC 5545-compliant data structures (VEvent, VTodo, etc.)
- Create simplified v2 API endpoints with direct DateTime support
- Eliminate ~150 lines of manual string parsing in handlers
- Add structured attendee and alarm support
- Maintain backward compatibility with existing v1 APIs

## Frontend Improvements
- Replace 16+ parameter create_event calls with single structured request
- Add automatic date/time conversion in EventCreationData
- Eliminate enum-to-string conversions throughout event creation
- Add v2 API models with proper type safety

## Technical Benefits
- Direct DateTime<Utc> usage instead of error-prone string parsing
- Proper RFC 5545 compliance with DTSTAMP, SEQUENCE fields
- Vec<AttendeeV2> instead of comma-separated strings
- Structured alarm system with multiple reminder types
- Enhanced RRULE support for complex recurrence patterns

## Code Quality
- Reduced create_event call from 16 parameters to 1 structured request
- Added comprehensive integration plan documentation
- Both backend and frontend compile successfully
- Maintained full backward compatibility during transition

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

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

backend/src/lib.rs

@@ -8,6 +8,7 @@ use std::sync::Arc;
 
 mod auth;
 mod models;
+mod models_v2;
 mod handlers;
 mod calendar;
 mod config;
@@ -44,6 +45,10 @@ pub async fn run_server() -> Result<(), Box<dyn std::error::Error>> {
         .route("/api/calendar/events/create", post(handlers::create_event))
         .route("/api/calendar/events/update", post(handlers::update_event))
         .route("/api/calendar/events/delete", post(handlers::delete_event))
+        // V2 API routes with better type safety
+        .route("/api/v2/calendar/events/create", post(handlers::create_event_v2))
+        .route("/api/v2/calendar/events/update", post(handlers::update_event_v2))
+        .route("/api/v2/calendar/events/delete", post(handlers::delete_event_v2))
         .route("/api/calendar/events/:uid", get(handlers::refresh_event))
         .layer(
             CorsLayer::new()