feat: Add post-processing completion tags & info

[galligan]

I would love to be able to see paperless-gpt add some info to my docs after the processing has finished up. In particular, it would be helpful to add tags to a document that's been processed.

For example, there could be a tag for each type of action, including if the job was done manually or automatically.

Finally, you could make use of Custom Fields to add rich metadata to the processed doc.

This might include:

• A permalink to the job record, so I can always go back to see what paperless-gpt did
• Job-specific info, such as models used, tokens consumed, etc.

I saw this type of feature was mentioned in #94 but it feels worthy enough to break out into its own issue to address.