chore(vps): register fleet-prometheus in topology and services

- hosts/vps/services.yaml: fleet-prometheus entry (role: fleet-liveness-source,
  exposure: tailscale-internal, port 9090, runtime paths, depends_on node_exporter)
- inventory/topology.yaml: add fleet-prometheus to the vps services list

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hosts/vps/services.yaml

@@ -41,3 +41,21 @@ services:
     depends_on:
       local: []
       external: []
+
+  fleet-prometheus:
+    role: fleet-liveness-source
+    deployment_model: docker-compose
+    exposure: tailscale-internal
+    offline_required: false
+    depends_on:
+      local:
+        - node_exporter
+      external: []
+    ports:
+      - name: http
+        container_port: 9090
+        protocol: tcp
+    runtime:
+      config_path: /opt/homelab/config/fleet-prometheus
+      data_path: /opt/homelab/data/fleet-prometheus
+      logs_path: /opt/homelab/logs/fleet-prometheus

inventory/topology.yaml

@@ -41,6 +41,7 @@ nodes:
       - node-agent
       - control-plane       # executor, observer, supervisor, operator-ui
       - node_exporter
+      - fleet-prometheus    # Fleet liveness source of truth (Tailscale-internal)
       - stability-agent
       - npm                 # Nginx Proxy Manager — public ingress, TLS termination
       - outline             # Team wiki (outline + postgres + redis)