Add logging stack: Loki + Alloy + Grafana for router/switch/phone syslog

Alloy receives syslog on 514/udp and 1514/tcp from network devices,
forwards to a local Loki instance; Grafana provisioned with Loki as
default datasource. Compose bind-mounts config from
/mnt/containers/logging/... on the host (staged separately) since
Portainer's GitOps pull only fetches the compose file, not the repo's
other files.

logging/loki-config.yaml

@@ -0,0 +1,47 @@
+auth_enabled: false
+
+server:
+  http_listen_port: 3100
+  grpc_listen_port: 9096
+  log_level: info
+
+common:
+  path_prefix: /loki
+  storage:
+    filesystem:
+      chunks_directory: /loki/chunks
+      rules_directory: /loki/rules
+  replication_factor: 1
+  ring:
+    instance_addr: 127.0.0.1
+    kvstore:
+      store: inmemory
+
+schema_config:
+  configs:
+    - from: 2024-01-01
+      store: tsdb
+      object_store: filesystem
+      schema: v13
+      index:
+        prefix: index_
+        period: 24h
+
+limits_config:
+  reject_old_samples: true
+  reject_old_samples_max_age: 168h
+  retention_period: 2160h  # 90 days — adjust to taste, disk on this host is ~42GB free
+  max_query_series: 5000
+
+compactor:
+  working_directory: /loki/compactor
+  compaction_interval: 10m
+  retention_enabled: true
+  retention_delete_delay: 2h
+  delete_request_store: filesystem
+
+ruler:
+  storage:
+    type: local
+    local:
+      directory: /loki/rules