inventory: declare systemd_unit for the underscore-named services

Service keys in inventory use _ for python-attribute friendliness but the
actual systemd units use dashes. Explicit systemd_unit field disambiguates
for MCP tail_log / get_service_status.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

inventory.yaml

@@ -80,6 +80,7 @@ services:
   homelab_mcp:
     backend: apps
     port: 9810
+    systemd_unit: homelab-mcp
     public_host: mcp.hubris.network
     endpoint: https://mcp.hubris.network/sse
     note: MCP server. Read-only context + management. Reachable on the LAN via Caddy
@@ -88,6 +89,7 @@ services:
   secrets_issuance:
     backend: apps
     port: 9820
+    systemd_unit: secrets-issuance
     public_host: secrets.hubris.network
     endpoint: https://secrets.hubris.network/issue
     note: Issues per-client age private keys. Gated at source-IP layer (mesh + LAN