GDPR-compliant retention and deletion for workflow/API uploaded files

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### 1. Is this request related to a challenge you're experiencing? Tell me about your story.

We are running Dify v1.16.0 as a self-hosted Docker deployment.

Our workflows process uploaded audio files, documents and images through the API. These files may contain personal data and must therefore be deleted after a defined retention period for GDPR compliance.

Currently, Dify provides upload and download/preview functionality, but no supported API endpoint to delete an uploaded file by its `upload_file_id`.

The `FILES_ACCESS_TIMEOUT` setting only invalidates signed URLs. It does not delete the physical file or its database metadata.

As a result, uploaded files accumulate in the Dify storage over time. This creates both a storage management problem and a GDPR compliance risk, because personal data may remain stored indefinitely.

### 2. Additional context or comments

Dify already provides the following administrative cleanup commands:

- `flask clear-orphaned-file-records`
- `flask remove-orphaned-files-on-storage`

These commands were introduced in PR #18835:

https://github.com/langgenius/dify/pull/18835

However, they only remove files that are already orphaned. They do not provide a retention policy for valid files that are still referenced by workflow executions, messages or execution logs.

Related issues:

- #38515 - Add retention policy and quota controls for workflow/API uploaded files:
https://github.com/langgenius/dify/issues/38515

- #27420 - Add API and GUI cleanup mechanism to delete uploaded files:
https://github.com/langgenius/dify/issues/27420

- #19528 - Delete workflow files:
https://github.com/langgenius/dify/issues/19528

We need a supported and documented GDPR-compliant cleanup mechanism with the following capabilities:

- Configurable retention period for Workflow/API uploaded files
- Automatic scheduled deletion of expired files
- Optional deletion immediately after successful workflow execution
- Manual administrative cleanup command or API
- Deletion of both the physical storage object and the database metadata
- Safe handling of files referenced by messages or workflow execution logs
- Dry-run mode, audit logging and retry handling
- Clear separation between temporary workflow uploads and permanent resources such as knowledge-base documents, app icons and avatars

A public endpoint such as the following would also be useful:

`DELETE /v1/files/{file_id}`

Invalidating a signed URL or removing only an orphaned database record is not sufficient for GDPR compliance. The actual stored file and all related personal-data metadata must be removed.

### 3. Can you help us with this feature?

- [x] I am interested in contributing to this feature.

Start with the existing `flask clear-orphaned-file-records` and `flask remove-orphaned-files-on-storage` administrative commands, then review related issues #38515, #27420, and #19528. Define the scope for workflow/API uploads separately from permanent resources, including scheduled or manual cleanup, storage objects and database metadata, references, dry runs, audit logs, and retries; done means the retention behavior is documented and safely verifiable.

Written by the indexing model from the issue text.