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/cdn_manager.md

@@ -0,0 +1,51 @@
+# Module: cdn_manager
+
+## Purpose
+Manages file upload and download to/from an S3-compatible CDN (MinIO/similar) using separate credentials for upload and download operations.
+
+## Public Interface
+
+### CDNCredentials
+| Field | Type | Description |
+|-------|------|-------------|
+| `host` | str | CDN endpoint URL |
+| `downloader_access_key` | str | S3 access key for downloads |
+| `downloader_access_secret` | str | S3 secret for downloads |
+| `uploader_access_key` | str | S3 access key for uploads |
+| `uploader_access_secret` | str | S3 secret for uploads |
+
+### CDNManager
+| Method | Signature | Returns | Description |
+|--------|-----------|---------|-------------|
+| `__init__` | `(credentials: CDNCredentials)` | — | Creates two boto3 S3 clients (download + upload) |
+| `upload` | `(bucket: str, filename: str, file_bytes: bytearray) -> bool` | True on success | Uploads bytes to S3 bucket |
+| `download` | `(bucket: str, filename: str) -> bool` | True on success | Downloads file from S3 to current directory |
+
+## Internal Logic
+- Maintains two separate boto3 S3 clients with different credentials (read vs write separation)
+- Upload uses `upload_fileobj` with in-memory BytesIO wrapper
+- Download uses `download_file` (saves directly to disk with same filename)
+- Both methods catch all exceptions, print error, return bool
+
+## Dependencies
+- `boto3` (external) — S3 client
+- `io`, `sys`, `yaml`, `os` (stdlib) — Note: `sys`, `yaml`, `os` are imported but unused
+
+## Consumers
+api_client, exports, train, start_inference
+
+## Data Models
+`CDNCredentials` — plain data class holding S3 access credentials.
+
+## Configuration
+Credentials loaded from `cdn.yaml` by callers (not by this module directly).
+
+## External Integrations
+- S3-compatible object storage (configured via `CDNCredentials.host`)
+
+## Security
+- Separate read/write credentials enforce least-privilege access
+- Credentials passed in at construction time, not hardcoded here
+
+## Tests
+None.