Automate the Grafana dashboard export step #100

The current loop is: edit a dashboard in the Grafana UI, copy the JSON out,
commit it. docs/roadmap.md is blunt about the consequence — it is "manual and
therefore skipped under pressure".

The provisioning is deliberately one-way: allowUiUpdates: false and
editable: false, with the reasoning inline in
provisioning/dashboards/dashboards.yaml:

Dashboards are version-controlled in this repository, so edits made in the
Grafana UI are intentionally discarded on restart.

That is the right design and it puts all the burden on the export step. A UI edit
that is not exported is silently destroyed on the next restart, which is a
correct outcome that feels like data loss.

What would help

A make dashboards-export that pulls each dashboard by uid from the Grafana
API, strips the volatile fields (id, version, iteration) and writes it
back over the file in grafana/dashboards/, so the loop becomes edit → one
command → git diff.

scripts/capture-screenshots.sh (#62) already does the authentication half of
this — Grafana admin credentials read in-process via secrets-env.sh and passed
to curl through --config — so there is a pattern to copy.

Worth pairing with a CI check that the committed JSON round-trips, otherwise the
export drifts from what Grafana would produce and every diff is noise.

Tracked in docs/roadmap.md; filed as an issue so it has a place to be discussed and closed.