Add file management API with local storage backend

- Implement FileHandler with CRUD operations for files/directories
- Add FileService with business logic and SHA-256 hashing
- Create LocalStorage backend for filesystem persistence
- Add database repository with pagination and name uniqueness
  constraints
- Configure max upload size in storage settings
- Include comprehensive tests for all layers

internal/config/config.go

@@ -39,8 +39,9 @@ type PostgresConfig struct {
 }
 
 type StorageConfig struct {
-	Driver string             `mapstructure:"driver"`
-	Local  LocalStorageConfig `mapstructure:"local"`
+	Driver        string             `mapstructure:"driver"`
+	Local         LocalStorageConfig `mapstructure:"local"`
+	MaxUploadSize int64              `mapstructure:"max_upload_size"`
 }
 
 type LocalStorageConfig struct {
@@ -90,6 +91,10 @@ func (c *Config) Validate() error {
 		errs = append(errs, errors.New("storage.local.path: must not be empty"))
 	}
 
+	if c.Storage.MaxUploadSize < 0 {
+		errs = append(errs, errors.New("storage.max_upload_size: must not be negative"))
+	}
+
 	if c.JWT.Secret == "" {
 		errs = append(errs, errors.New("jwt.secret: must not be empty"))
 	}