Add external calendars feature: display read-only ICS calendars alongside CalDAV calendars

- Database: Add external_calendars table with user relationships and CRUD operations
- Backend: Implement REST API endpoints for external calendar management and ICS fetching
- Frontend: Add external calendar modal, sidebar section with visibility toggles
- Calendar integration: Merge external events with regular events in unified view
- ICS parsing: Support multiple datetime formats, recurring events, and timezone handling
- Authentication: Integrate with existing JWT token system for user-specific calendars
- UI: Visual distinction with 📅 indicator and separate management section

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

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

backend/src/handlers/mod.rs

@@ -0,0 +1,15 @@
+pub mod auth;
+pub mod calendar;
+pub mod events;
+pub mod external_calendars;
+pub mod ics_fetcher;
+pub mod preferences;
+pub mod series;
+
+pub use auth::*;
+pub use calendar::*;
+pub use events::*;
+pub use external_calendars::*;
+pub use ics_fetcher::*;
+pub use preferences::*;
+pub use series::*;