Add Python ffmpeg parallel-workload tutorial sample

Python/Batch/article_samples/ffmpeg/README.md

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+## Azure Batch Python ffmpeg Article Sample
+
+A Python application that uses Azure Batch to process media files in parallel
+with the [ffmpeg](https://ffmpeg.org/) open-source tool. The sample converts
+MP4 media files to MP3 format, in parallel, across a pool of Batch compute
+nodes.
+
+For details and explanation, see the accompanying article
+[Tutorial: Run a parallel workload with Azure Batch using the Python API](https://learn.microsoft.com/azure/batch/tutorial-parallel-python).
+
+### Prerequisites
+
+- [Python 3.8 or later](https://www.python.org/downloads/) including `pip`
+- An Azure Batch account and a linked general-purpose Azure Storage account
+- The [Azure CLI](https://learn.microsoft.com/cli/azure/install-azure-cli)
+
+### Authentication
+
+This sample authenticates to Azure Batch and Azure Storage with Microsoft
+Entra ID through
+[`DefaultAzureCredential`](https://learn.microsoft.com/python/api/azure-identity/azure.identity.defaultazurecredential).
+**No account keys are used.**
+
+Sign in and make sure your identity has the required roles on both accounts:
+
+```bash
+az login
+```
+
+- On the **Batch account**, assign a role that allows data-plane operations,
+  such as **Azure Batch Data Contributor** (required to create pools, jobs,
+  and tasks).
+- On the **Storage account**, assign **Storage Blob Data Contributor**
+  (required to create containers, upload input files, and request the user
+  delegation key used to sign SAS URLs).
+
+```bash
+# Batch account role
+az role assignment create \
+    --assignee "<your-user-principal-name>" \
+    --role "Azure Batch Data Contributor" \
+    --scope "/subscriptions/<subscription-id>/resourceGroups/<resource-group>/providers/Microsoft.Batch/batchAccounts/<batch-account-name>"
+
+# Storage account role
+az role assignment create \
+    --assignee "<your-user-principal-name>" \
+    --role "Storage Blob Data Contributor" \
+    --scope "/subscriptions/<subscription-id>/resourceGroups/<resource-group>/providers/Microsoft.Storage/storageAccounts/<storage-account-name>"
+```
+
+> Role assignments can take a few minutes to propagate.
+
+### Setting up the Python environment
+
+Install the required packages using the
+[requirements.txt](./requirements.txt) file:
+
+```bash
+pip install -r requirements.txt
+```
+
+The sample uses the current Azure SDK for Python libraries:
+[`azure-batch`](https://pypi.org/project/azure-batch/) (`BatchClient`),
+[`azure-identity`](https://pypi.org/project/azure-identity/), and
+[`azure-storage-blob`](https://pypi.org/project/azure-storage-blob/).
+
+### Running the sample
+
+1. Open [config.py](./config.py) and set the values unique to your accounts
+   (no keys required):
+
+   ```python
+   _BATCH_ACCOUNT_NAME = 'yourbatchaccount'
+   _BATCH_ACCOUNT_URL = 'https://yourbatchaccount.yourregion.batch.azure.com'
+   _STORAGE_ACCOUNT_NAME = 'mystorageaccount'
+   ```
+
+2. Run the app:
+
+   ```bash
+   python batch_python_tutorial_ffmpeg.py
+   ```
+
+By default the sample creates a pool of five Spot (low-priority) nodes of size
+`STANDARD_A1_v2` running **Ubuntu Server 24.04 LTS**. You can change the pool
+size, VM size, and node counts in `config.py`.
+
+> Marketplace VM images and Batch node agents have support end dates. To list
+> the image references and node agent SKUs that your Batch account currently
+> supports, call the
+> [`list_supported_images`](https://learn.microsoft.com/python/api/azure-batch/azure.batch.batchclient#azure-batch-batchclient-list-supported-images)
+> method.