Refactor constants management to use Pydantic BaseModel for configuration

- Replaced module-level path variables in constants.py with a structured Pydantic Config class.
- Updated all relevant modules (train.py, augmentation.py, exports.py, dataset-visualiser.py, manual_run.py) to access paths through the new config structure.
- Fixed bugs related to image processing and model saving.
- Enhanced test infrastructure to accommodate the new configuration approach.

This refactor improves code maintainability and clarity by centralizing configuration management.

_docs/02_document/modules/dataset_visualiser.md

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+# Module: dataset-visualiser
+
+## Purpose
+Interactive tool for visually inspecting annotated images from datasets or the processed folder, displaying bounding boxes with class colors.
+
+## Public Interface
+
+| Function | Signature | Description |
+|----------|-----------|-------------|
+| `visualise_dataset` | `()` | Iterates images in a specific dataset folder, shows each with annotations. Waits for keypress. |
+| `visualise_processed_folder` | `()` | Shows images from the processed folder with annotations. |
+
+## Internal Logic
+- **visualise_dataset()**: Hardcoded to a specific dataset date (`2024-06-18`), iterates from index 35247 onward. Reads image + labels, calls `ImageLabel.visualize()`, waits for user input to advance.
+- **visualise_processed_folder()**: Lists all processed images, shows the first one.
+- Both functions use `read_labels()` imported from a `preprocessing` module **which does not exist** in the codebase — this is a broken import.
+
+## Dependencies
+- `constants` — directory paths (datasets_dir, prefix, processed_*)
+- `dto/annotationClass` — AnnotationClass for class colors
+- `dto/imageLabel` — ImageLabel for visualization
+- `preprocessing` — **MISSING MODULE** (read_labels function)
+- `cv2` (external), `matplotlib` (external), `os`, `pathlib` (stdlib)
+
+## Consumers
+None (standalone script).
+
+## Data Models
+Uses ImageLabel, AnnotationClass.
+
+## Configuration
+Hardcoded dataset path and start index.
+
+## External Integrations
+Filesystem I/O, matplotlib interactive display.
+
+## Security
+None.
+
+## Tests
+None.