grafana: provision Loki/Prometheus datasources + log dashboards

Bind two ready-to-use dashboards at grafana.t4t-gateway.com:

- Logs — All services: per-service log volume timeseries + live tail,
  with a service selector and a free-text search box.
- Hosted-gateway API — errors & PSS inbox: error rate, ack/deliver
  counters, and a live tail filtered to PSS inbox traffic so the
  webhook fallback is observable.

Datasources + dashboard providers are mounted via Docker configs from
hostinger/grafana/ so they survive grafana container restarts and live
in git rather than the Grafana UI.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

hostinger/grafana/dashboards.yaml

@@ -0,0 +1,13 @@
+apiVersion: 1
+
+providers:
+  - name: t4t
+    orgId: 1
+    folder: ''
+    type: file
+    disableDeletion: false
+    updateIntervalSeconds: 30
+    allowUiUpdates: true
+    options:
+      path: /etc/grafana/dashboards
+      foldersFromFilesStructure: false