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-156_test_label_validation.md

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+# Label Validation Tests
+
+**Task**: AZ-156_test_label_validation
+**Name**: Label Validation Tests
+**Description**: Implement 5 blackbox tests for YOLO label validation — valid labels, out-of-range coords, missing files, multi-line corruption
+**Complexity**: 1 point
+**Dependencies**: AZ-152_test_infrastructure
+**Component**: Blackbox Tests
+**Jira**: AZ-156
+**Epic**: AZ-151
+
+## Problem
+
+Labels must be validated before dataset formation. Tests verify the check_label function correctly accepts valid labels and rejects corrupted ones.
+
+## Outcome
+
+- 5 passing pytest tests in `tests/test_label_validation.py`
+
+## Scope
+
+### Included
+- BT-LBL-01: Valid label accepted (returns True)
+- BT-LBL-02: Label with x > 1.0 rejected (returns False)
+- BT-LBL-03: Label with height > 1.0 rejected (returns False)
+- BT-LBL-04: Missing label file rejected (returns False)
+- BT-LBL-05: Multi-line label with one corrupted line (returns False)
+
+### Excluded
+- Integration with dataset formation (separate task)
+
+## Acceptance Criteria
+
+**AC-1: Valid label**
+Given label file with content `0 0.5 0.5 0.1 0.1`
+When check_label(path) is called
+Then returns True
+
+**AC-2: x out of range**
+Given label file with content `0 1.5 0.5 0.1 0.1`
+When check_label(path) is called
+Then returns False
+
+**AC-3: height out of range**
+Given label file with content `0 0.5 0.5 0.1 1.2`
+When check_label(path) is called
+Then returns False
+
+**AC-4: Missing file**
+Given non-existent file path
+When check_label(path) is called
+Then returns False
+
+**AC-5: Multi-line corruption**
+Given label with `0 0.5 0.5 0.1 0.1\n3 0.5 0.5 0.1 1.5`
+When check_label(path) is called
+Then returns False
+
+## Constraints
+
+- Label files are generated in tmp_path at test time
+- No external fixtures needed