Evaluate live-data indexer architecture and providers

## Outcome

A later architecture decision determines whether and how GovTool should introduce a dedicated live-data indexer.

## Scope

- Compare db-sync, Blockfrost, Kupo, and Ogmios for live-data needs
- Identify latency, completeness, operational, and failure-mode requirements
- Define boundaries between provider adapters, indexing, caching, and API services
- Produce an ADR and follow-up options without committing implementation

## 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 comparing db-sync, Blockfrost, Kupo, and Ogmios against the stated latency, completeness, operational, and failure-mode requirements. Define the boundaries between provider adapters, indexing, caching, and API services, then produce the ADR, split independently deliverable child issues, and link the resulting architecture and API decisions.

Written by the indexing model from the issue text.