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/styles.css

@@ -3816,6 +3816,37 @@ body {
     white-space: nowrap;
 }
 
+.external-calendar-actions {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+    flex-shrink: 0;
+}
+
+.last-updated {
+    font-size: 0.7rem;
+    color: rgba(255, 255, 255, 0.6);
+    opacity: 0.8;
+}
+
+.external-calendar-refresh-btn {
+    background: none;
+    border: none;
+    color: rgba(255, 255, 255, 0.7);
+    cursor: pointer;
+    font-size: 0.8rem;
+    padding: 2px 4px;
+    border-radius: 3px;
+    transition: all 0.2s ease;
+    line-height: 1;
+}
+
+.external-calendar-refresh-btn:hover {
+    color: rgba(255, 255, 255, 0.9);
+    background: rgba(255, 255, 255, 0.1);
+    transform: rotate(180deg);
+}
+
 .external-calendar-indicator {
     font-size: 0.8rem;
     opacity: 0.7;