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/db.rs

@@ -99,6 +99,38 @@ pub struct UserPreferences {
     pub updated_at: DateTime<Utc>,
 }
 
+/// External calendar model
+#[derive(Debug, Clone, Serialize, Deserialize, FromRow)]
+pub struct ExternalCalendar {
+    pub id: i32,
+    pub user_id: String,
+    pub name: String,
+    pub url: String,
+    pub color: String,
+    pub is_visible: bool,
+    pub created_at: DateTime<Utc>,
+    pub updated_at: DateTime<Utc>,
+    pub last_fetched: Option<DateTime<Utc>>,
+}
+
+impl ExternalCalendar {
+    /// Create a new external calendar
+    pub fn new(user_id: String, name: String, url: String, color: String) -> Self {
+        let now = Utc::now();
+        Self {
+            id: 0, // Will be set by database
+            user_id,
+            name,
+            url,
+            color,
+            is_visible: true,
+            created_at: now,
+            updated_at: now,
+            last_fetched: None,
+        }
+    }
+}
+
 impl UserPreferences {
     /// Create default preferences for a new user
     pub fn default_for_user(user_id: String) -> Self {
@@ -308,6 +340,91 @@ impl<'a> PreferencesRepository<'a> {
         .execute(self.db.pool())
         .await?;
 
+        Ok(())
+    }
+}
+
+/// Repository for ExternalCalendar operations
+pub struct ExternalCalendarRepository<'a> {
+    db: &'a Database,
+}
+
+impl<'a> ExternalCalendarRepository<'a> {
+    pub fn new(db: &'a Database) -> Self {
+        Self { db }
+    }
+
+    /// Get all external calendars for a user
+    pub async fn get_by_user(&self, user_id: &str) -> Result<Vec<ExternalCalendar>> {
+        sqlx::query_as::<_, ExternalCalendar>(
+            "SELECT * FROM external_calendars WHERE user_id = ? ORDER BY created_at ASC",
+        )
+        .bind(user_id)
+        .fetch_all(self.db.pool())
+        .await
+    }
+
+    /// Create a new external calendar
+    pub async fn create(&self, calendar: &ExternalCalendar) -> Result<i32> {
+        let result = sqlx::query(
+            "INSERT INTO external_calendars (user_id, name, url, color, is_visible, created_at, updated_at) 
+             VALUES (?, ?, ?, ?, ?, ?, ?)",
+        )
+        .bind(&calendar.user_id)
+        .bind(&calendar.name)
+        .bind(&calendar.url)
+        .bind(&calendar.color)
+        .bind(&calendar.is_visible)
+        .bind(&calendar.created_at)
+        .bind(&calendar.updated_at)
+        .execute(self.db.pool())
+        .await?;
+
+        Ok(result.last_insert_rowid() as i32)
+    }
+
+    /// Update an external calendar
+    pub async fn update(&self, id: i32, calendar: &ExternalCalendar) -> Result<()> {
+        sqlx::query(
+            "UPDATE external_calendars 
+             SET name = ?, url = ?, color = ?, is_visible = ?, updated_at = ?
+             WHERE id = ? AND user_id = ?",
+        )
+        .bind(&calendar.name)
+        .bind(&calendar.url)
+        .bind(&calendar.color)
+        .bind(&calendar.is_visible)
+        .bind(Utc::now())
+        .bind(id)
+        .bind(&calendar.user_id)
+        .execute(self.db.pool())
+        .await?;
+
+        Ok(())
+    }
+
+    /// Delete an external calendar
+    pub async fn delete(&self, id: i32, user_id: &str) -> Result<()> {
+        sqlx::query("DELETE FROM external_calendars WHERE id = ? AND user_id = ?")
+            .bind(id)
+            .bind(user_id)
+            .execute(self.db.pool())
+            .await?;
+
+        Ok(())
+    }
+
+    /// Update last_fetched timestamp
+    pub async fn update_last_fetched(&self, id: i32, user_id: &str) -> Result<()> {
+        sqlx::query(
+            "UPDATE external_calendars SET last_fetched = ? WHERE id = ? AND user_id = ?",
+        )
+        .bind(Utc::now())
+        .bind(id)
+        .bind(user_id)
+        .execute(self.db.pool())
+        .await?;
+
         Ok(())
     }
 }