Increase comment max length to match post max length (server + client)

Summary

Comments have a different (shorter) maximum length than posts, which is inconsistent and limits useful discussion. The max length of comments should be updated to match the posts' length on both server and client sides.

Steps to Reproduce

Not provided.

Expected Behaviour

Comments should have the same maximum length as posts.

Actual Behaviour

Comments have a shorter maximum length than posts.

Environment

Not provided.

Additional Context

The fix needs to be applied in:

• Server-side validators (comment creation/editing)
• Client-side character counter/validator
• Database schema if there is a column constraint

There is a related issue about AI correction/modifier content length limits also needing to be increased (mentioned in the same conversation).

Reported by retoor via DevPlace.