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/inference_onnx_engine.md

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+# Module: inference/onnx_engine
+
+## Purpose
+Defines the abstract `InferenceEngine` base class and the `OnnxEngine` implementation for running ONNX model inference with GPU acceleration.
+
+## Public Interface
+
+### InferenceEngine (ABC)
+| Method | Signature | Description |
+|--------|-----------|-------------|
+| `__init__` | `(model_path: str, batch_size: int = 1, **kwargs)` | Abstract constructor |
+| `get_input_shape` | `() -> Tuple[int, int]` | Returns (height, width) of model input |
+| `get_batch_size` | `() -> int` | Returns the batch size |
+| `run` | `(input_data: np.ndarray) -> List[np.ndarray]` | Runs inference, returns output tensors |
+
+### OnnxEngine (extends InferenceEngine)
+| Method | Signature | Description |
+|--------|-----------|-------------|
+| `__init__` | `(model_bytes, batch_size: int = 1, **kwargs)` | Loads ONNX model from bytes, creates InferenceSession with CUDA+CPU providers |
+| `get_input_shape` | `() -> Tuple[int, int]` | Returns (height, width) from model input shape |
+| `get_batch_size` | `() -> int` | Returns batch size (from model shape or constructor arg) |
+| `run` | `(input_data: np.ndarray) -> List[np.ndarray]` | Runs ONNX inference session |
+
+## Internal Logic
+- Uses ONNX Runtime with `CUDAExecutionProvider` (primary) and `CPUExecutionProvider` (fallback).
+- Reads model metadata to extract class names from custom metadata map.
+- If model input shape has a fixed batch dimension (not -1), overrides the constructor batch_size.
+
+## Dependencies
+- `onnxruntime` (external) — ONNX inference runtime
+- `numpy` (external)
+- `abc`, `typing` (stdlib)
+
+## Consumers
+inference/inference, inference/tensorrt_engine (inherits InferenceEngine), train (imports OnnxEngine)
+
+## Data Models
+None.
+
+## Configuration
+None.
+
+## External Integrations
+- ONNX Runtime GPU execution (CUDA)
+
+## Security
+None.
+
+## Tests
+None.