Fix recurring event series modification via drag and drop operations

This commit resolves the "Failed to fetch" errors when updating recurring
event series through drag operations by implementing proper request
sequencing and fixing time parameter handling.

Key fixes:
- Eliminate HTTP request cancellation by sequencing operations properly
- Add global mutex to prevent CalDAV HTTP race conditions
- Implement complete RFC 5545-compliant series splitting for "this_and_future"
- Fix frontend to pass dragged times instead of original times
- Add comprehensive error handling and request timing logs
- Backend now handles both UPDATE (add UNTIL) and CREATE (new series) in single request

Technical changes:
- Frontend: Remove concurrent CREATE request, pass dragged times to backend
- Backend: Implement full this_and_future logic with sequential operations
- CalDAV: Add mutex serialization and detailed error tracking
- Series: Create new series with occurrence date + dragged times

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

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

backend/Cargo.toml

@@ -32,6 +32,7 @@ regex = "1.0"
 dotenvy = "0.15"
 base64 = "0.21"
 thiserror = "1.0"
+lazy_static = "1.4"
 
 [dev-dependencies]
 tokio = { version = "1.0", features = ["macros", "rt"] }