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-162_test_nms.md

@@ -0,0 +1,50 @@
+# NMS Overlap Removal Tests
+
+**Task**: AZ-162_test_nms
+**Name**: NMS Overlap Removal Tests
+**Description**: Implement 3 tests for non-maximum suppression — overlapping kept by confidence, non-overlapping preserved, chain overlap resolution
+**Complexity**: 1 point
+**Dependencies**: AZ-152_test_infrastructure
+**Component**: Blackbox Tests
+**Jira**: AZ-162
+**Epic**: AZ-151
+
+## Problem
+
+The NMS module removes overlapping detections based on IoU threshold (0.3), keeping the higher-confidence detection. Tests verify all overlap scenarios.
+
+## Outcome
+
+- 3 passing pytest tests in `tests/test_nms.py`
+
+## Scope
+
+### Included
+- BT-NMS-01: Overlapping detections — keep higher confidence (IoU > 0.3 → 1 kept)
+- BT-NMS-02: Non-overlapping detections — keep both (IoU < 0.3 → 2 kept)
+- BT-NMS-03: Chain overlap resolution (A↔B, B↔C → ≤ 2 kept)
+
+### Excluded
+- Integration with inference pipeline (separate task)
+
+## Acceptance Criteria
+
+**AC-1: Overlap removal**
+Given 2 Detections at same position, confidence 0.9 and 0.5, IoU > 0.3
+When remove_overlapping_detections() runs
+Then 1 detection returned (confidence 0.9)
+
+**AC-2: Non-overlapping preserved**
+Given 2 Detections at distant positions, IoU < 0.3
+When remove_overlapping_detections() runs
+Then 2 detections returned
+
+**AC-3: Chain overlap**
+Given 3 Detections: A overlaps B, B overlaps C, A doesn't overlap C
+When remove_overlapping_detections() runs
+Then ≤ 2 detections; highest confidence per overlapping pair kept
+
+## Constraints
+
+- Detection objects constructed in-memory (no fixture files)
+- IoU threshold is 0.3 (from constants or hardcoded in NMS)