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/config/config.go

@@ -20,8 +20,22 @@ type ServerConfig struct {
 }
 
 type DatabaseConfig struct {
-	Driver string `mapstructure:"driver"`
-	Path   string `mapstructure:"path"`
+	Driver   string         `mapstructure:"driver"`
+	SQLite   SQLiteConfig   `mapstructure:"sqlite"`
+	Postgres PostgresConfig `mapstructure:"postgres"`
+}
+
+type SQLiteConfig struct {
+	Path string `mapstructure:"path"`
+}
+
+type PostgresConfig struct {
+	Host     string `mapstructure:"host"`
+	Port     int    `mapstructure:"port"`
+	User     string `mapstructure:"user"`
+	Password string `mapstructure:"password"`
+	DBName   string `mapstructure:"dbname"`
+	SSLMode  string `mapstructure:"sslmode"`
 }
 
 type StorageConfig struct {
@@ -60,8 +74,26 @@ func (c *Config) Validate() error {
 		errs = append(errs, fmt.Errorf("server.host: %q is not a valid IP address", c.Server.Host))
 	}
 
-	if c.Database.Path == "" {
-		errs = append(errs, errors.New("database.path: must not be empty"))
+	switch c.Database.Driver {
+	case "sqlite3":
+		if c.Database.SQLite.Path == "" {
+			errs = append(errs, errors.New("database.sqlite.path: must not be empty"))
+		}
+	case "postgres":
+		if c.Database.Postgres.Host == "" {
+			errs = append(errs, errors.New("database.postgres.host: must not be empty"))
+		}
+		if c.Database.Postgres.Port < 1 || c.Database.Postgres.Port > 65535 {
+			errs = append(errs, fmt.Errorf("database.postgres.port: %d out of range [1, 65535]", c.Database.Postgres.Port))
+		}
+		if c.Database.Postgres.User == "" {
+			errs = append(errs, errors.New("database.postgres.user: must not be empty"))
+		}
+		if c.Database.Postgres.DBName == "" {
+			errs = append(errs, errors.New("database.postgres.dbname: must not be empty"))
+		}
+	default:
+		errs = append(errs, fmt.Errorf("database.driver: %q is not supported (use sqlite3 or postgres)", c.Database.Driver))
 	}
 
 	if c.Storage.Local.Path == "" {