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/datasources.yaml

@@ -0,0 +1,18 @@
+apiVersion: 1
+
+datasources:
+  - name: Loki
+    uid: loki
+    type: loki
+    access: proxy
+    url: http://loki:3100
+    isDefault: true
+    editable: false
+    jsonData:
+      maxLines: 5000
+  - name: Prometheus
+    uid: prometheus
+    type: prometheus
+    access: proxy
+    url: http://prometheus:9090
+    editable: false