[grafana] Schema v39 requires Grafana 10.4+ — compose stack and README import instructions do not document minimum version

[obchain]

Refs #54

Location

deploy/grafana/charon.json (schemaVersion: 39), README.md (3-step import)

Problem

Grafana schema version 39 was introduced in Grafana 10.4.x. Importing a schema v39 dashboard into Grafana 9.x fails with an error or silently drops panels. The PR description mentions the version requirement but the README import instructions contain no minimum version constraint.

The sibling PR (#55, docker-compose) is stacked on this branch. If the compose file pins grafana/grafana:latest or a 9.x tag, the import will fail. The Hetzner CX22 quickstart in the README does not specify which Grafana version to use.

Impact

Operators running Grafana 9.x (common on self-hosted deployments that have not updated in the last 18 months) will get a failed import with no explanation. The README gives no diagnostic path.

Suggested Fix

1. Add to the README import section: minimum Grafana 10.4.x required.
2. Ensure the docker-compose stack (PR feat(deploy): Dockerfile + compose stack for Hetzner CX22 #55) pins grafana/grafana at 10.4.x or later with a specific digest.
3. Optionally downgrade to schema v36 (Grafana 10.0+) if the v39-specific features are not required.

[obchain]

Fixed on feature branch; see feat/22-prometheus-metrics / feat/26-grafana-dashboard commits.