Improve DRep profile and activity APIs

## Outcome

Consumers can obtain richer DRep profiles and a reliable chronological activity history.

## Scope

- Define the canonical DRep profile and activity models
- Include registration and update history, delegations, voting activity, and past votes
- Expose vote rationales and relevant contextual governance data
- Define pagination, ordering, freshness, and missing-data behavior

## Acceptance criteria

- [ ] Scope is decomposed into independently deliverable child issues.
- [ ] Architecture and API decisions are linked from this issue.
- [ ] Test, documentation, observability, rollout, and failure behavior are defined where applicable.
- [ ] Epic exit criteria are verified before closure.

## Planning note

This is a 2026 GovTool revamp work item. Its child features and tasks should be added as the scope is refined.

Start by decomposing the requested DRep profile and activity capabilities into independently deliverable child issues. Define and link the architecture and API decisions, including models, pagination, ordering, freshness, missing-data behavior, testing, documentation, observability, rollout, and failure behavior. Done means the epic exit criteria are verified before closure.

Written by the indexing model from the issue text.