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-163_test_annotation_queue.md

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+# Annotation Queue Message Tests
+
+**Task**: AZ-163_test_annotation_queue
+**Name**: Annotation Queue Message Tests
+**Description**: Implement 5 tests for annotation queue message parsing — Created, Validated bulk, Deleted bulk, malformed handling
+**Complexity**: 2 points
+**Dependencies**: AZ-152_test_infrastructure
+**Component**: Blackbox Tests
+**Jira**: AZ-163
+**Epic**: AZ-151
+
+## Problem
+
+The annotation queue processes msgpack-encoded messages from RabbitMQ Streams. Tests must verify correct parsing of all message types and graceful handling of malformed input.
+
+## Outcome
+
+- 5 passing pytest tests in `tests/test_annotation_queue.py`
+
+## Scope
+
+### Included
+- BT-AQM-01: Parse Created annotation message (all fields populated correctly)
+- BT-AQM-02: Parse Validated bulk message (status == Validated, names list matches)
+- BT-AQM-03: Parse Deleted bulk message (status == Deleted, names list matches)
+- BT-AQM-04: Malformed message raises exception
+- RT-AQM-01: Malformed msgpack bytes handled (exception caught, no crash)
+
+### Excluded
+- Live RabbitMQ Streams connection (requires external service)
+- Queue offset persistence (requires live broker)
+
+## Acceptance Criteria
+
+**AC-1: Created message**
+Given msgpack bytes matching AnnotationMessage schema (status=Created, role=Validator)
+When decoded and constructed
+Then all fields populated: name, detections, image bytes, status == "Created", role == "Validator"
+
+**AC-2: Validated bulk**
+Given msgpack bytes with status=Validated, list of names
+When decoded and constructed
+Then status == "Validated", names list matches input
+
+**AC-3: Deleted bulk**
+Given msgpack bytes with status=Deleted, list of names
+When decoded and constructed
+Then status == "Deleted", names list matches input
+
+**AC-4: Malformed msgpack**
+Given invalid msgpack bytes
+When decode is attempted
+Then exception raised
+
+**AC-5: Resilient handling**
+Given random bytes (not valid msgpack)
+When passed to message handler
+Then exception caught, handler doesn't crash
+
+## Constraints
+
+- Msgpack messages constructed in-memory at test time
+- Must match the AnnotationMessage/AnnotationBulkMessage schemas from annotation-queue/
+- Resilience test marked: `@pytest.mark.resilience`