Add intelligent caching and auto-refresh for external calendars

Implements server-side database caching with 5-minute refresh intervals to
dramatically improve external calendar performance while keeping data fresh.

Backend changes:
- New external_calendar_cache table with ICS data storage
- Smart cache logic: serves from cache if < 5min old, fetches fresh otherwise
- Cache repository methods for get/update/clear operations
- Migration script for cache table creation

Frontend changes:
- 5-minute auto-refresh interval for background updates
- Manual refresh button (🔄) for each external calendar
- Last updated timestamps showing when each calendar was refreshed
- Centralized refresh function with proper cleanup on logout

Performance improvements:
- Initial load: instant from cache vs slow external HTTP requests
- Background updates: fresh data without user waiting
- Reduced external API calls: only when cache is stale
- Scalable: handles multiple external calendars efficiently

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

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

frontend/src/services/calendar_service.rs

@@ -2089,6 +2089,7 @@ impl CalendarService {
         #[derive(Deserialize)]
         struct ExternalCalendarEventsResponse {
             events: Vec<VEvent>,
+            last_fetched: chrono::DateTime<chrono::Utc>,
         }
 
         let response: ExternalCalendarEventsResponse = serde_wasm_bindgen::from_value(json)