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

internal/app/webapp_test.go

@@ -9,7 +9,7 @@ import (
 func TestNewWebApp(t *testing.T) {
 	cfg := &config.Config{}
 
-	webApp := NewWebApp(cfg)
+	webApp := NewWebApp(cfg, nil, nil, nil, nil)
 
 	if webApp.Config != cfg {
 		t.Fatal("Config was not assigned")
@@ -18,3 +18,10 @@ func TestNewWebApp(t *testing.T) {
 		t.Errorf("Version = %q, want %q", webApp.Version, AppVersion)
 	}
 }
+
+func TestCloseNilDB(t *testing.T) {
+	webApp := NewWebApp(&config.Config{}, nil, nil, nil, nil)
+	if err := webApp.Close(); err != nil {
+		t.Errorf("Close with nil DB should not error: %v", err)
+	}
+}