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-158_test_model_split.md

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+# Model Split Tests
+
+**Task**: AZ-158_test_model_split
+**Name**: Model Split Tests
+**Description**: Implement 2 blackbox tests for model split storage — size constraint and reassembly integrity
+**Complexity**: 1 point
+**Dependencies**: AZ-152_test_infrastructure
+**Component**: Blackbox Tests
+**Jira**: AZ-158
+**Epic**: AZ-151
+
+## Problem
+
+Encrypted models are split into small and big parts for CDN storage. Tests must verify the split respects size constraints and reassembly produces the original.
+
+## Outcome
+
+- 2 passing pytest tests in `tests/test_model_split.py`
+
+## Scope
+
+### Included
+- BT-SPL-01: Split respects size constraint (small ≤ max(3072 bytes, 20% of total))
+- BT-SPL-02: Reassembly produces original (small + big == encrypted bytes)
+
+### Excluded
+- CDN upload/download (requires external service)
+
+## Acceptance Criteria
+
+**AC-1: Size constraint**
+Given 10000 encrypted bytes
+When split into small + big
+Then small ≤ max(3072, total × 0.2); big = remainder
+
+**AC-2: Reassembly**
+Given split parts from 10000 encrypted bytes
+When small + big concatenated
+Then equals original encrypted bytes
+
+## Constraints
+
+- Uses generated binary data (no fixture files needed)
+- References SMALL_SIZE_KB constant from constants.py