Fix /metrics double-gzip + deploy script for amd64 build

The Echo gzip middleware was wrapping promhttp's pre-gzipped output, so
vmagent received double-compressed bytes that failed the Prometheus
parser with binary garbage. Skipping /metrics in the gzip Skipper.

Three deploy-script fixes uncovered while shipping this:
- _config.sh had backticks around \"kubectl get cm\" inside the python
  heredoc, which bash treated as command substitution when KUBECONFIG
  was set. Quoted the literal instead.
- 03-deploy.sh now passes --platform linux/amd64 to all docker builds
  so arm64 Macs don't push images that fail with \"exec format error\"
  on the Hetzner CX nodes.
- OBS_INGEST_TOKEN lookup was reading deploy-k3s/prod.env instead of
  the actual deploy/prod.env at the repo root.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

deploy-k3s/scripts/_config.sh

@@ -145,7 +145,7 @@ lines = [
     # (set by 02-setup-secrets.sh). Wire them into the api/worker
     # deployments via envFrom: secretRef when B2 uploads need to be
     # active. Leaving them in cleartext here would leak via
-    # `kubectl get cm`.
+    # \"kubectl get cm\".
     f\"B2_BUCKET_NAME={val(st['b2_bucket'])}\",
     f\"B2_ENDPOINT={val(st['b2_endpoint'])}\",
     f\"B2_REGION={val(st.get('b2_region'))}\",