Replaces tpaulus/tmobile-internet-exporter upstream image, which crashes
on every scrape against the G4AR because the gateway returns `bars` as a
JSON float (`4.0`) and the upstream struct types it as int.
The vendored exporter:
- decodes all radio numerics as float64 so `4.0` round-trips
- treats the 4g / 5g signal blocks as optional (the G4AR omits 4g)
- evicts stale (type, band) series when a band disappears between scrapes
- adds a scrape_errors_total counter and /healthz endpoint
Deployment now pulls git.mcintire.me/graham/tmobile-exporter:v1 via the
forgejo-registry imagePullSecret (matching the convention used by aprs,
prop, towerops). Build + push the image before Argo can pull it; the
imagePullSecret must also be copied into the monitoring namespace.