feat: migrate stack to Portainer git-managed, rename project to valhalla-lab

- Rename project from htpc-download-box to valhalla-lab (directory + dc alias)
- Remove portainer and watchtower from compose; run standalone outside managed stack
  to prevent self-termination during Portainer-triggered redeployments
- Add portainer_proxy external network shared by caddy and standalone portainer
  so Caddy can route to Portainer via Docker DNS without them being in the same stack
- Carve out POST /api/stacks/webhooks/* in Caddyfile so Portainer webhooks can
  reach through the internal_only guard without exposing the full UI
- Update apply-compose.ps1 and sync-prod.ps1 paths from htpc-download-box to valhalla-lab

Caddyfile

@@ -185,8 +185,19 @@ dev.ginnoir.com {
 # MANAGEMENT — internal only
 # =============================================================
 portainer.ginnoir.com {
-	import internal_only
-	reverse_proxy portainer:9000
+	@webhook {
+		method POST
+		path /api/stacks/webhooks/*
+	}
+
+	handle @webhook {
+		reverse_proxy portainer:9000
+	}
+
+	handle {
+		import internal_only
+		reverse_proxy portainer:9000
+	}
 }
 
 uptime.ginnoir.com {