Allow removal of original PDF once all parties signed

Is your feature request related to a problem? Please describe.
Context:
I create a Code of conduct that 10 people need to sign, separately/independently.

Steps:

1. Convert a random odt file to PDF (10 times)
2. Send a signature request to each person (10 times)
3. Await signatures (10 times... - sorry, I had to 😂 )
4. See 10 original files and 10 signed files. 😮‍💨
5. Make a confused and overwhelmed face 😕 and delete the original files because you are already lost in your tidy folder system with thousands of files, and you don't know where to put them
6. Realize you shouldn't have deleted the 10 original files 😱
7. Create a GitHub issue 🏆

Describe the solution you'd like
Once signed, the user should have the possibility to remove the original document if not needed.

Describe alternatives you've considered

Additional context
https://github.com/LibreSign/libresign/issues/4160#issuecomment-2548723509

This is a good feature. Now the original document is associated to the signed document by a database relation and we have logic flows that is associated to this and I think that the main is that now, when delete the original document we delete the signed document. Other very important flow that I think that will be the most hard to change is because the UUID to validate the document is an attribute of original file and not of signed file and have a lot of side effects to change this. Will be necessary change these flows.

We also need to think what's the best moment to allow to delete the original document. Now, the document .signed is created when the first signer sign the document. Thinking at scenario that we have 3 signers, always that a signer see the document to sign, is displayed the original document and not the document with the signature of previous signers. To remove the association from original document and signed document, will be necessary change this and show, by example, to 2th signer the version with the signature of 1th signer and the 3th signer will see the document already signed by the 1th and 2th signer. This is a good point of improvement, but we need to think if have security impacts.

Could you create a new issue specific to this? I think that this could be described at a new issue.

No files or tests are named. Start by tracing the original-to-signed document database relation, the original file's UUID validation flow, and deletion behavior described in the issue. Done means the original can be removed at an appropriate signing stage without breaking validation, while later signers see the correct cumulative signed version and security impacts are addressed.