fix(alloy): promote CONTAINER_NAME to an indexed Loki stream label

Loki metrics queries group only by indexed stream labels, so the
journal-as-JSON stream made every per-container view structurally
empty (measured 6.5k lines/5min flowing while the board read zero).
A loki.process relabel stage now extracts CONTAINER_NAME into a
'container' label. alloy 1.21's json stage has no on_error attribute
(validated against the pinned image) — format_as_json guarantees the
line shape, so no error branch exists.

shared/alloy/config.alloy

@@ -17,6 +17,9 @@ logging {
 // Loki is published on the host's loopback (127.0.0.1:3100). Alloy shares the
 // host network, so it pushes by loopback, never by container name — a root
 // container is not on the rootless dankclaw_default network.
+// GRAFANA_HOST_LABEL is replaced with this host's name by
+// copy_alloy_config at deploy time; it must not appear verbatim in the
+// installed config.
 loki.write "default" {
   endpoint {
     url = "http://127.0.0.1:3100/loki/api/v1/push"
@@ -32,7 +35,34 @@ loki.write "default" {
 loki.source.journal "journal" {
   format_as_json = true
   labels = {
-    job = "systemd-journal",
+    job  = "systemd-journal",
+    host = "GRAFANA_HOST_LABEL",
+  }
+  forward_to = [loki.process.relabel.receiver]
+}
+
+// Promote the fields that ARE the point of the log stream to real Loki labels.
+// Without this the journal arrives as one opaque JSON line per entry and every
+// per-container view is structurally impossible: Loki metrics queries can only
+// group by INDEXED stream labels, so `sum by (container_name)` returns a single
+// anonymous series and the logs-overview board read empty beside 6.5k
+// lines/5min of live traffic (measured 2026-10-21). CONTAINER_NAME is what
+// podman's journald logging writes (uppercase); host units lack it and stay on
+// the host-only stream. `format_as_json = true` above guarantees every line
+// reaching this stage is a JSON object, so there is no non-JSON branch to
+// protect — alloy 1.21's json stage has no `on_error` attribute (validated
+// against the pinned image); a malformed line would drop, which is the
+// correct fate for a line Loki could not index anyway.
+loki.process "relabel" {
+  stage.json {
+    expressions = {
+      container = "CONTAINER_NAME",
+    }
+  }
+  stage.labels {
+    values = {
+      container = "container",
+    }
   }
   forward_to = [loki.write.default.receiver]
 }