feat(grafana): provision the Loki datasource

Adds shared/grafana/provisioning/datasources/loki.yml at http://loki:3100
so logs are readable through Grafana Explore. Loki ships no web UI of its
own and the guarded vhost serves only the API; this is the human surface.

shared/grafana/provisioning/datasources/loki.yml

@@ -0,0 +1,17 @@
+# Grafana datasource for Loki, declared in git rather than clicked into
+# grafana.db — same reason as the Prometheus datasource: a UI-configured
+# datasource is invisible to review, lost with the data volume, and unknown
+# to the repository. Loki OSS ships no web UI of its own, so Explore through
+# this datasource is the only way a human reads the log store.
+#
+# URL is the container name on dankclaw_default (the same address Caddy
+# proxies to), not the published loopback port.
+apiVersion: 1
+
+datasources:
+  - name: Loki
+    uid: loki
+    type: loki
+    access: proxy
+    url: http://loki:3100
+    editable: false