Implement lightweight auth system with SQLite

Added SQLite database for session management and user preferences storage,
allowing users to have consistent settings across different sessions and devices.

Backend changes:
- Added SQLite database with users, sessions, and preferences tables
- Implemented session-based authentication alongside JWT tokens
- Created preference storage/retrieval API endpoints
- Database migrations for schema setup
- Session validation and cleanup functionality

Frontend changes:
- Added "Remember server" and "Remember username" checkboxes to login
- Created preferences service for syncing settings with backend
- Updated auth flow to handle session tokens and preferences
- Store remembered values in LocalStorage (not database) for convenience

Key features:
- User preferences persist across sessions and devices
- CalDAV passwords never stored, only passed through
- Sessions expire after 24 hours
- Remember checkboxes only affect local browser storage

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

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

backend/src/lib.rs

@@ -9,27 +9,37 @@ use tower_http::cors::{Any, CorsLayer};
 pub mod auth;
 pub mod calendar;
 pub mod config;
+pub mod db;
 pub mod handlers;
 pub mod models;
 
 use auth::AuthService;
+use db::Database;
 
 #[derive(Clone)]
 pub struct AppState {
     pub auth_service: AuthService,
+    pub db: Database,
 }
 
 pub async fn run_server() -> Result<(), Box<dyn std::error::Error>> {
     // Initialize logging
     println!("🚀 Starting Calendar Backend Server");
 
+    // Initialize database
+    let database_url = std::env::var("DATABASE_URL")
+        .unwrap_or_else(|_| "sqlite:calendar.db".to_string());
+    
+    let db = Database::new(&database_url).await?;
+    println!("✅ Database initialized");
+
     // Create auth service
     let jwt_secret = std::env::var("JWT_SECRET")
         .unwrap_or_else(|_| "your-super-secret-jwt-key-change-in-production".to_string());
 
-    let auth_service = AuthService::new(jwt_secret);
+    let auth_service = AuthService::new(jwt_secret, db.clone());
 
-    let app_state = AppState { auth_service };
+    let app_state = AppState { auth_service, db };
 
     // Build our application with routes
     let app = Router::new()
@@ -58,6 +68,10 @@ pub async fn run_server() -> Result<(), Box<dyn std::error::Error>> {
             "/api/calendar/events/series/delete",
             post(handlers::delete_event_series),
         )
+        // User preferences endpoints
+        .route("/api/preferences", get(handlers::get_preferences))
+        .route("/api/preferences", post(handlers::update_preferences))
+        .route("/api/auth/logout", post(handlers::logout))
         .layer(
             CorsLayer::new()
                 .allow_origin(Any)