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`

config.example.yaml

@@ -22,6 +22,11 @@ storage:
     # Max upload file size in bytes. 0  = unlimited
     max_upload_size: 104857600 # 100 MB
 
+log:
+    level: info        # terminal: debug, info, warn, error
+    file_path: ""      # empty = no file logging
+    file_level: debug  # file: debug, info, warn, error
+
 jwt:
     secret: change-me-in-production
     access_ttl: 15m