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/auth.rs

@@ -1,10 +1,12 @@
 use chrono::{Duration, Utc};
 use jsonwebtoken::{decode, encode, Algorithm, DecodingKey, EncodingKey, Header, Validation};
 use serde::{Deserialize, Serialize};
+use uuid::Uuid;
 
 use crate::calendar::CalDAVClient;
 use crate::config::CalDAVConfig;
-use crate::models::{ApiError, AuthResponse, CalDAVLoginRequest};
+use crate::db::{Database, PreferencesRepository, Session, SessionRepository, UserRepository};
+use crate::models::{ApiError, AuthResponse, CalDAVLoginRequest, UserPreferencesResponse};
 
 #[derive(Debug, Serialize, Deserialize)]
 pub struct Claims {
@@ -17,11 +19,12 @@ pub struct Claims {
 #[derive(Clone)]
 pub struct AuthService {
     jwt_secret: String,
+    db: Database,
 }
 
 impl AuthService {
-    pub fn new(jwt_secret: String) -> Self {
-        Self { jwt_secret }
+    pub fn new(jwt_secret: String, db: Database) -> Self {
+        Self { jwt_secret, db }
     }
 
     /// Authenticate user directly against CalDAV server
@@ -49,13 +52,47 @@ impl AuthService {
                     "✅ Authentication successful! Found {} calendars",
                     calendars.len()
                 );
-                // Authentication successful, generate JWT token
-                let token = self.generate_token(&request.username, &request.server_url)?;
-
+                
+                // Find or create user in database
+                let user_repo = UserRepository::new(&self.db);
+                let user = user_repo
+                    .find_or_create(&request.username, &request.server_url)
+                    .await
+                    .map_err(|e| ApiError::Database(format!("Failed to create user: {}", e)))?;
+                
+                // Generate JWT token
+                let jwt_token = self.generate_token(&request.username, &request.server_url)?;
+                
+                // Generate session token
+                let session_token = format!("sess_{}", Uuid::new_v4());
+                
+                // Create session in database
+                let session = Session::new(user.id.clone(), session_token.clone(), 24);
+                let session_repo = SessionRepository::new(&self.db);
+                session_repo
+                    .create(&session)
+                    .await
+                    .map_err(|e| ApiError::Database(format!("Failed to create session: {}", e)))?;
+                
+                // Get or create user preferences
+                let prefs_repo = PreferencesRepository::new(&self.db);
+                let preferences = prefs_repo
+                    .get_or_create(&user.id)
+                    .await
+                    .map_err(|e| ApiError::Database(format!("Failed to get preferences: {}", e)))?;
+                
                 Ok(AuthResponse {
-                    token,
+                    token: jwt_token,
+                    session_token,
                     username: request.username,
                     server_url: request.server_url,
+                    preferences: UserPreferencesResponse {
+                        calendar_selected_date: preferences.calendar_selected_date,
+                        calendar_time_increment: preferences.calendar_time_increment,
+                        calendar_view_mode: preferences.calendar_view_mode,
+                        calendar_theme: preferences.calendar_theme,
+                        calendar_colors: preferences.calendar_colors,
+                    },
                 })
             }
             Err(err) => {
@@ -143,4 +180,33 @@ impl AuthService {
 
         Ok(token_data.claims)
     }
+    
+    /// Validate session token
+    pub async fn validate_session(&self, session_token: &str) -> Result<String, ApiError> {
+        let session_repo = SessionRepository::new(&self.db);
+        
+        let session = session_repo
+            .find_by_token(session_token)
+            .await
+            .map_err(|e| ApiError::Database(format!("Failed to find session: {}", e)))?
+            .ok_or_else(|| ApiError::Unauthorized("Invalid session token".to_string()))?;
+        
+        if session.is_expired() {
+            return Err(ApiError::Unauthorized("Session expired".to_string()));
+        }
+        
+        Ok(session.user_id)
+    }
+    
+    /// Logout user by deleting session
+    pub async fn logout(&self, session_token: &str) -> Result<(), ApiError> {
+        let session_repo = SessionRepository::new(&self.db);
+        
+        session_repo
+            .delete(session_token)
+            .await
+            .map_err(|e| ApiError::Database(format!("Failed to delete session: {}", e)))?;
+        
+        Ok(())
+    }
 }