Persist and cache governance metadata

## Outcome

Fetched governance metadata is persisted and served through an explicit, observable cache lifecycle.

## Scope

- Define storage schema and metadata lifecycle states
- Persist source, fetch time, validation result, and diagnostic details
- Define cache keys, expiry, invalidation, and reprocessing behavior
- Provide migration and backfill tooling

## 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.

No files, tests, or entry points are named. Start by decomposing the storage, lifecycle, cache, migration, and backfill scope into child issues, then link the architecture and API decisions. Done means the epic has independently deliverable tasks and verified test, documentation, observability, rollout, and failure-behavior criteria.

Written by the indexing model from the issue text.