Add a staging values overlay distinct from the dev install

## Summary
Add a staging `values` overlay distinct from the bare dev install — the chart currently ships exactly one `values.yaml` for every environment.

## Source of truth
`rebuild-roadmap.md`, "The cluster track" (R6 row).

## Current state
Verified: `charts/tigerbeetle/values.yaml` is the only values file in the chart (`charts/tigerbeetle/` contains `Chart.lock`, `Chart.yaml`, `templates/`, `values.yaml` — no `values-staging.yaml` or overlay directory). `replicas: 3` is hardcoded under `controllers.main`, with an inline comment: *"In the current version of TigerBeetle, cluster size can not be changed after creation, but this limitation will be lifted in the future."* Three replicas is correct as TigerBeetle's consensus minimum, but there is currently no dev/staging distinction in resource sizing, storage class, or any other environment-specific setting — one shape serves every environment.

## Acceptance criteria
- A staging values overlay exists, distinct from the default/dev `values.yaml`, covering at minimum whatever differs in practice between a throwaway dev instance and a longer-lived staging cluster (storage class/size, resource requests/limits, or other environment-appropriate settings as identified during the work).
- `replicas: 3` stays the consensus minimum in both — this ticket is about environment-specific config, not cluster size.

## Dependencies
None; independent of the backup/restore and upgrade-path tickets, though all three are R6 chart-hardening work and may be convenient to batch.

## Out of scope
A full multi-environment (dev/staging/prod) matrix — only the dev-vs-staging split named in the roadmap.