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/repository/db.go

@@ -0,0 +1,57 @@
+package repository
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"gorm.io/driver/postgres"
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+
+	"github.com/dhao2001/mygo/internal/config"
+	"github.com/dhao2001/mygo/internal/model"
+)
+
+// Open creates a GORM database connection based on the config driver.
+func Open(cfg config.DatabaseConfig) (*gorm.DB, error) {
+	var dialector gorm.Dialector
+
+	switch cfg.Driver {
+	case "sqlite3":
+		dir := filepath.Dir(cfg.SQLite.Path)
+		if err := os.MkdirAll(dir, 0755); err != nil {
+			return nil, fmt.Errorf("create db directory: %w", err)
+		}
+		dialector = sqlite.Open(cfg.SQLite.Path)
+	case "postgres":
+		dsn := fmt.Sprintf(
+			"host=%s user=%s password=%s dbname=%s port=%d sslmode=%s",
+			cfg.Postgres.Host,
+			cfg.Postgres.User,
+			cfg.Postgres.Password,
+			cfg.Postgres.DBName,
+			cfg.Postgres.Port,
+			cfg.Postgres.SSLMode,
+		)
+		dialector = postgres.Open(dsn)
+	default:
+		return nil, fmt.Errorf("unsupported database driver: %s", cfg.Driver)
+	}
+
+	db, err := gorm.Open(dialector, &gorm.Config{})
+	if err != nil {
+		return nil, fmt.Errorf("open database: %w", err)
+	}
+
+	return db, nil
+}
+
+// AutoMigrate runs schema migration for all domain models.
+func AutoMigrate(db *gorm.DB) error {
+	return db.AutoMigrate(
+		&model.User{},
+		&model.Session{},
+		&model.File{},
+	)
+}