Add structured logging and centralized error handling

- Initialize slog in the serve command with terminal/file support
- Introduce `AppError` with HTTP status for unified service-layer errors
- Replace ad-hoc `api.Error` calls with `api.RespondError`
- Wrap internal errors with `model.NewInternalError` and add reference
  IDs
- Add `RequestID` middleware and switch router from `gin.Default` to
  `gin.New`

internal/app/webapp.go

@@ -2,6 +2,7 @@ package app
 
 import (
 	"fmt"
+	"log/slog"
 
 	"gorm.io/gorm"
 
@@ -56,7 +57,7 @@ func Bootstrap(cfg *config.Config) (*WebApp, error) {
 		return nil, fmt.Errorf("init storage: %w", err)
 	}
 
-	fileService := service.NewFileService(fileRepo, store, cfg.Storage.MaxUploadSize)
+	fileService := service.NewFileService(fileRepo, store, cfg.Storage.MaxUploadSize, slog.Default())
 
 	return &WebApp{
 		Config:         cfg,