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/lib.rs

@@ -1,6 +1,6 @@
 use axum::{
     response::Json,
-    routing::{get, post},
+    routing::{delete, get, post},
     Router,
 };
 use std::sync::Arc;
@@ -72,6 +72,12 @@ pub async fn run_server() -> Result<(), Box<dyn std::error::Error>> {
         .route("/api/preferences", get(handlers::get_preferences))
         .route("/api/preferences", post(handlers::update_preferences))
         .route("/api/auth/logout", post(handlers::logout))
+        // External calendars endpoints
+        .route("/api/external-calendars", get(handlers::get_external_calendars))
+        .route("/api/external-calendars", post(handlers::create_external_calendar))
+        .route("/api/external-calendars/:id", post(handlers::update_external_calendar))
+        .route("/api/external-calendars/:id", delete(handlers::delete_external_calendar))
+        .route("/api/external-calendars/:id/events", get(handlers::fetch_external_calendar_events))
         .layer(
             CorsLayer::new()
                 .allow_origin(Any)