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/manual_run.md

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+# Module: manual_run
+
+## Purpose
+Ad-hoc script for manual training operations. Contains commented-out alternatives and a hardcoded workflow for copying model weights and exporting.
+
+## Public Interface
+No functions or classes. Script-level code only.
+
+## Internal Logic
+- Contains commented-out calls to `Augmentator().augment_annotations()`, `train.train_dataset()`, `train.resume_training()`.
+- Active code: references a specific model date (`2025-05-18`), removes intermediate epoch checkpoint files, copies `best.pt` to `CURRENT_PT_MODEL`, then calls `train.export_current_model()`.
+- Serves as a developer convenience script for one-off training/export operations.
+
+## Dependencies
+- `constants` — models_dir, prefix, CURRENT_PT_MODEL
+- `train` — export_current_model
+- `augmentation` — Augmentator (imported, usage commented out)
+- `glob`, `os`, `shutil` (stdlib)
+
+## Consumers
+None (standalone script).
+
+## Data Models
+None.
+
+## Configuration
+Hardcoded model date: `2025-05-18`.
+
+## External Integrations
+Filesystem operations on `/azaion/models/`.
+
+## Security
+None.
+
+## Tests
+None.