chore: replace Promtail with Grafana Alloy for log collection (refs #97)

Promtail 2.9.0 embeds Docker client API v1.42 which is incompatible with
Docker Engine v29 (minimum API v1.44). Grafana Alloy v1.12.2 resolves this
by using a compatible Docker client.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/alloy/config.alloy

@@ -0,0 +1,29 @@
+discovery.docker "containers" {
+  host             = "unix:///var/run/docker.sock"
+  refresh_interval = "5s"
+}
+
+discovery.relabel "containers" {
+  targets = discovery.docker.containers.targets
+  rule {
+    source_labels = ["__meta_docker_container_name"]
+    regex         = "/(.*)"
+    target_label  = "container"
+  }
+  rule {
+    source_labels = ["__meta_docker_container_label_com_docker_compose_service"]
+    target_label  = "service"
+  }
+}
+
+loki.source.docker "containers" {
+  host       = "unix:///var/run/docker.sock"
+  targets    = discovery.relabel.containers.output
+  forward_to = [loki.write.default.receiver]
+}
+
+loki.write "default" {
+  endpoint {
+    url = "http://mvp-loki:3100/loki/api/v1/push"
+  }
+}