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>

backend/migrations/007_create_external_calendar_cache_table.sql

@@ -0,0 +1,14 @@
+-- Create external calendar cache table for storing ICS data
+CREATE TABLE external_calendar_cache (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    external_calendar_id INTEGER NOT NULL,
+    ics_data TEXT NOT NULL,
+    cached_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    etag TEXT,
+    FOREIGN KEY (external_calendar_id) REFERENCES external_calendars(id) ON DELETE CASCADE,
+    UNIQUE(external_calendar_id)
+);
+
+-- Index for faster lookups
+CREATE INDEX idx_external_calendar_cache_calendar_id ON external_calendar_cache(external_calendar_id);
+CREATE INDEX idx_external_calendar_cache_cached_at ON external_calendar_cache(cached_at);

backend/src/db.rs

@@ -1,4 +1,4 @@
-use chrono::{DateTime, Utc};
+use chrono::{DateTime, Duration, Utc};
 use serde::{Deserialize, Serialize};
 use sqlx::sqlite::{SqlitePool, SqlitePoolOptions};
 use sqlx::{FromRow, Result};
@@ -427,4 +427,62 @@ impl<'a> ExternalCalendarRepository<'a> {
 
         Ok(())
     }
+
+    /// Get cached ICS data for an external calendar
+    pub async fn get_cached_data(&self, external_calendar_id: i32) -> Result<Option<(String, DateTime<Utc>)>> {
+        let result = sqlx::query_as::<_, (String, DateTime<Utc>)>(
+            "SELECT ics_data, cached_at FROM external_calendar_cache WHERE external_calendar_id = ?",
+        )
+        .bind(external_calendar_id)
+        .fetch_optional(self.db.pool())
+        .await?;
+
+        Ok(result)
+    }
+
+    /// Update cache with new ICS data
+    pub async fn update_cache(&self, external_calendar_id: i32, ics_data: &str, etag: Option<&str>) -> Result<()> {
+        sqlx::query(
+            "INSERT INTO external_calendar_cache (external_calendar_id, ics_data, etag, cached_at)
+             VALUES (?, ?, ?, ?)
+             ON CONFLICT(external_calendar_id) DO UPDATE SET
+             ics_data = excluded.ics_data,
+             etag = excluded.etag,
+             cached_at = excluded.cached_at",
+        )
+        .bind(external_calendar_id)
+        .bind(ics_data)
+        .bind(etag)
+        .bind(Utc::now())
+        .execute(self.db.pool())
+        .await?;
+
+        Ok(())
+    }
+
+    /// Check if cache is stale (older than max_age_minutes)
+    pub async fn is_cache_stale(&self, external_calendar_id: i32, max_age_minutes: i64) -> Result<bool> {
+        let cutoff_time = Utc::now() - Duration::minutes(max_age_minutes);
+        
+        let result = sqlx::query_scalar::<_, i64>(
+            "SELECT COUNT(*) FROM external_calendar_cache 
+             WHERE external_calendar_id = ? AND cached_at > ?",
+        )
+        .bind(external_calendar_id)
+        .bind(cutoff_time)
+        .fetch_one(self.db.pool())
+        .await?;
+
+        Ok(result == 0)
+    }
+
+    /// Clear cache for an external calendar
+    pub async fn clear_cache(&self, external_calendar_id: i32) -> Result<()> {
+        sqlx::query("DELETE FROM external_calendar_cache WHERE external_calendar_id = ?")
+            .bind(external_calendar_id)
+            .execute(self.db.pool())
+            .await?;
+
+        Ok(())
+    }
 }

backend/src/handlers/ics_fetcher.rs

@@ -53,35 +53,69 @@ pub async fn fetch_external_calendar_events(
         }));
     }
 
-    // Fetch ICS content from URL
-    let client = Client::new();
-    let response = client
-        .get(&calendar.url)
-        .send()
-        .await
-        .map_err(|e| ApiError::Internal(format!("Failed to fetch calendar: {}", e)))?;
+    // Check cache first
+    let cache_max_age_minutes = 5;
+    let mut ics_content = String::new();
+    let mut last_fetched = Utc::now();
+    let mut fetched_from_cache = false;
 
-    if !response.status().is_success() {
-        return Err(ApiError::Internal(format!("Calendar server returned: {}", response.status())));
+    // Try to get from cache if not stale
+    match repo.is_cache_stale(id, cache_max_age_minutes).await {
+        Ok(is_stale) => {
+            if !is_stale {
+                // Cache is fresh, use it
+                if let Ok(Some((cached_data, cached_at))) = repo.get_cached_data(id).await {
+                    ics_content = cached_data;
+                    last_fetched = cached_at;
+                    fetched_from_cache = true;
+                }
+            }
+        }
+        Err(_) => {
+            // If cache check fails, proceed to fetch from URL
+        }
     }
 
-    let ics_content = response
-        .text()
-        .await
-        .map_err(|e| ApiError::Internal(format!("Failed to read calendar content: {}", e)))?;
+    // If not fetched from cache, get from external URL
+    if !fetched_from_cache {
+        let client = Client::new();
+        let response = client
+            .get(&calendar.url)
+            .send()
+            .await
+            .map_err(|e| ApiError::Internal(format!("Failed to fetch calendar: {}", e)))?;
+
+        if !response.status().is_success() {
+            return Err(ApiError::Internal(format!("Calendar server returned: {}", response.status())));
+        }
+
+        ics_content = response
+            .text()
+            .await
+            .map_err(|e| ApiError::Internal(format!("Failed to read calendar content: {}", e)))?;
+
+        // Store in cache for future requests
+        let etag = None; // TODO: Extract ETag from response headers if available
+        if let Err(e) = repo.update_cache(id, &ics_content, etag).await {
+            // Log error but don't fail the request
+            eprintln!("Failed to update cache for calendar {}: {}", id, e);
+        }
+
+        // Update last_fetched timestamp
+        if let Err(e) = repo.update_last_fetched(id, &user.id).await {
+            eprintln!("Failed to update last_fetched for calendar {}: {}", id, e);
+        }
+        
+        last_fetched = Utc::now();
+    }
 
     // Parse ICS content
     let events = parse_ics_content(&ics_content)
         .map_err(|e| ApiError::BadRequest(format!("Failed to parse calendar: {}", e)))?;
 
-    // Update last_fetched timestamp
-    repo.update_last_fetched(id, &user.id)
-        .await
-        .map_err(|e| ApiError::Database(format!("Failed to update last_fetched: {}", e)))?;
-
     Ok(Json(ExternalCalendarEventsResponse {
         events,
-        last_fetched: Utc::now(),
+        last_fetched,
     }))
 }