[Feature Request] Add description fields to metadata entities for AI-assisted classification

Description I would like to propose adding an optional description or context field to metadata entities such as: • Tags • Correspondents • Document types • Potentially storage paths This request is related to #371, which proposed adding more fields to correspondents, tags, and document types. That discussion was eventually closed due to inactivity, but I believe the introduction of AI-assisted classification creates a new and more specific use case for such a field. The goal is not primarily to turn Paperless-ngx into a CRM. Instead, the description would provide semantic context that can be used by humans, external integrations, workflows, and AI-based classifiers. --- Entity names are often too short or ambiguous to communicate when an entity should be assigned. For example, a Paperless-ngx instance may contain tags such as: • Taxes • Private • Insurance • Contract • Important A classifier can see…