Enable UTxO-HD for node 10.7.x and validate sync/load behavior

## Objective
Deliver a 10.7.x UTxO-HD rollout path for Rosetta operations.

## Scope
- Use PR #668 (LMDB attempt) as historical context/reference for prior implementation work.
- Confirm LSM + full Mithril support status for the 10.7.x backend path before enabling.
- Apply required configuration/version changes for UTxO-HD on 10.7.x.
- Validate sync completion and sync time after enabling.
- Validate indexer progression and core API sanity (`/network/status`, `/account/balance`, `/block`).
- Run load tests and compare results against current baseline.
- Record results in the issue.

## Done when
- Implementation changes are merged.
- Validation evidence is attached in the issue (including sync time and load test results).