feat(tags): add unique constraint to value and implement __str__

[DarshanCode2005]

This PR improves data integrity and observability for the Tag model.
Changes:
src/shared/models/cve.py :

• Added unique=True to the Tag.value field to prevent duplicate data.
• Implemented str to return the tag's value, improving the representation in logs.

Rationale: Duplicate tags ("critical", "critical" etc.) were previously possible in the database, leading to inconsistent data. This change enforces a clean tag registry. The string representation also significantly improves the developer & admin experience by making objects identifiable by their actual text.

[fricklerhandwerk]

This is one of the many things we can't "just do". At the very least it would require a migration to actually deduplicate the existing data, and a change in the ingestion code to do get_or_create. But what we just the other day happened to discuss with @adekoder is that we probably want to inline that extra table into the one field that references it. There's no use case currently in sight that would benefit from a separate table, and the amount of duplication an ArrayField would incur is negligible in the grand scheme of things.

[DarshanCode2005]

Thanks for the detailed explanation, that makes sense. I agree that adding unique=True would require a data migration and is not a small change anymore. I will remove that from this PR and keep it focused on the str improvement.

We can revisit uniqueness and possible schema simplification in a separate discussion/PR.

[fricklerhandwerk]

Now, were we doing that by hand anywhere in the code? If so, please use this new thing there, if not, we can close the PR. :)

[DarshanCode2005]

cve_reference.html

code snippet:

      {% for tag in ref.tags %}
        <span class="tag tag-gray">{{tag.value}}</span>
      {% endfor %}

so there we can use str(tag).

[fricklerhandwerk]

so there we can use str(tag).

Or just {{tag}} since string coercion is done implicitly?

[DarshanCode2005]

so there we can use str(tag).

Or just {{tag}} since string coercion is done implicitly?

Hmm.... I actually wrote that by mistake.