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/components/sidebar.rs

@@ -1,5 +1,6 @@
 use crate::components::CalendarListItem;
 use crate::services::calendar_service::{UserInfo, ExternalCalendar};
+use chrono::{DateTime, Local, TimeZone, Utc};
 use web_sys::HtmlSelectElement;
 use yew::prelude::*;
 use yew_router::prelude::*;
@@ -105,6 +106,7 @@ pub struct SidebarProps {
     pub external_calendars: Vec<ExternalCalendar>,
     pub on_external_calendar_toggle: Callback<i32>,
     pub on_external_calendar_delete: Callback<i32>,
+    pub on_external_calendar_refresh: Callback<i32>,
     pub color_picker_open: Option<String>,
     pub on_color_change: Callback<(String, String)>,
     pub on_color_picker_toggle: Callback<String>,
@@ -277,7 +279,36 @@ pub fn sidebar(props: &SidebarProps) -> Html {
                                                         style={format!("background-color: {}", cal.color)}
                                                     />
                                                     <span class="external-calendar-name">{&cal.name}</span>
-                                                    <span class="external-calendar-indicator"></span>
+                                                    <div class="external-calendar-actions">
+                                                        {
+                                                            if let Some(last_fetched) = cal.last_fetched {
+                                                                let local_time = last_fetched.with_timezone(&chrono::Local);
+                                                                html! {
+                                                                    <span class="last-updated" title={format!("Last updated: {}", local_time.format("%Y-%m-%d %H:%M"))}>
+                                                                        {format!("{}", local_time.format("%H:%M"))}
+                                                                    </span>
+                                                                }
+                                                            } else {
+                                                                html! {
+                                                                    <span class="last-updated">{"Never"}</span>
+                                                                }
+                                                            }
+                                                        }
+                                                        <button 
+                                                            class="external-calendar-refresh-btn" 
+                                                            title="Refresh calendar"
+                                                            onclick={{
+                                                                let on_refresh = props.on_external_calendar_refresh.clone();
+                                                                let cal_id = cal.id;
+                                                                Callback::from(move |e: MouseEvent| {
+                                                                    e.stop_propagation();
+                                                                    on_refresh.emit(cal_id);
+                                                                })
+                                                            }}
+                                                        >
+                                                            {"🔄"}
+                                                        </button>
+                                                    </div>
                                                 </div>
                                                 {
                                                     if *external_context_menu_open == Some(cal.id) {