refactor: extract MdrsClient service layer for library portability

To improve the tool's portability as a Python library, the core logic
has been decoupled from the CLI interface. This allows developers to
programmatically interact with MDRS without relying on CLI-specific
argument parsing or local file-based caches.

- Introduce `MdrsClient` service layer to handle core operations.
- Abstract authentication state using `CacheInterface` and `InMemoryCache`.
- Migrate all CLI commands to utilize `MdrsClient` for execution.
- Separate `Doi` data model from API responses and move to `models/doi.py`.
- Update `README.md` to include Python API usage examples.
- Bump package version to 1.3.17.

README.md

@@ -8,7 +8,7 @@ The mdrs-client-python is python library and a command-line client for up- and d
 poetry install
 ```
 
-## Example Usage
+## CLI Usage
 
 ### config create
 
@@ -205,3 +205,28 @@ Show the help message and exit
 ```shell
 mdrs -h
 ```
+
+## Python API Usage
+
+You can also use this package as a Python library to programmatically interact with MDRS repositories.
+
+```python
+from mdrsclient.client import MdrsClient
+from mdrsclient.cache import InMemoryCache
+
+# 1. Setup client with an in-memory cache to avoid local `.mdrsclient` state files
+cache = InMemoryCache()
+client = MdrsClient.from_remote("neurodata", cache=cache)
+
+# 2. Login to the remote server
+client.login("username", "password")
+
+# 3. Use service methods
+labs = client.get_laboratories()
+metadata = client.metadata("neurodata:/NIU/Repository/")
+
+# Transfer files programmatically
+client.upload("/path/to/local/data", "neurodata:/NIU/Repository/TEST/", is_recursive=True)
+client.download("neurodata:/NIU/Repository/TEST/data", "/path/to/local", is_recursive=True)
+```
+