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_tasks/AZ-159_test_annotation_classes.md

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+# Annotation Class & YAML Tests
+
+**Task**: AZ-159_test_annotation_classes
+**Name**: Annotation Class & YAML Tests
+**Description**: Implement 4 tests for annotation class loading, weather mode expansion, YAML generation, and total class count
+**Complexity**: 2 points
+**Dependencies**: AZ-152_test_infrastructure
+**Component**: Blackbox Tests
+**Jira**: AZ-159
+**Epic**: AZ-151
+
+## Problem
+
+The system loads 17 base annotation classes, expands them across 3 weather modes, and generates a data.yaml with 80 class slots. Tests verify the class pipeline.
+
+## Outcome
+
+- 4 passing pytest tests in `tests/test_annotation_classes.py`
+
+## Scope
+
+### Included
+- BT-CLS-01: Load 17 base classes from classes.json
+- BT-CLS-02: Weather mode expansion (offsets 0, 20, 40)
+- BT-CLS-03: YAML generation produces nc: 80 with 17 named + 63 placeholders
+- RL-CLS-01: Total class count is exactly 80
+
+### Excluded
+- Training configuration (beyond scope)
+
+## Acceptance Criteria
+
+**AC-1: Base classes**
+Given classes.json
+When AnnotationClass.read_json() is called
+Then returns dict with 17 unique base class entries
+
+**AC-2: Weather expansion**
+Given classes.json
+When classes are read
+Then same class exists at offset 0 (Norm), 20 (Wint), 40 (Night)
+
+**AC-3: YAML generation**
+Given classes.json + dataset path
+When create_yaml() runs with patched paths
+Then data.yaml contains nc: 80, 17 named classes + 63 Class-N placeholders
+
+**AC-4: Total count**
+Given classes.json
+When generating class list
+Then exactly 80 entries
+
+## Constraints
+
+- Uses classes.json from project root (fixture_classes_json)
+- YAML output goes to tmp_path
+- Resource limit test marked: `@pytest.mark.resource_limit`