Complete foundational data layer with repository implementation

- Add GORM dependencies for SQLite and PostgreSQL
- Create domain models (User, Session, File) with common errors
- Implement repository interfaces and database layer with migrations
- Update WebApp to bootstrap with database and repositories
- Add comprehensive unit tests for repository methods
- Update config structure to support multiple database drivers
- Extend AGENTS.md with debugging principles and dependency rules

cmd/serve.go

@@ -29,7 +29,16 @@ var serveCmd = &cobra.Command{
 		ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 		defer stop()
 
-		webApp := app.NewWebApp(cfg)
+		webApp, err := app.Bootstrap(cfg)
+		if err != nil {
+			return fmt.Errorf("bootstrap: %w", err)
+		}
+		defer func() {
+			if err := webApp.Close(); err != nil {
+				fmt.Fprintf(os.Stderr, "close webapp: %v\n", err)
+			}
+		}()
+
 		router := server.NewRouter(webApp)
 		addr := server.Address(webApp.Config.Server)