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

sync-prod.ps1

@@ -15,8 +15,8 @@ $repoDir = $PSScriptRoot
 
 Write-Host "Syncing production configs from $server ..."
 
-scp "${server}:~/htpc-download-box/docker-compose.yml" "$repoDir\docker-compose.yml"
-scp "${server}:~/htpc-download-box/.env"               "$repoDir\.env"
+scp "${server}:~/valhalla-lab/docker-compose.yml" "$repoDir\docker-compose.yml"
+scp "${server}:~/valhalla-lab/.env"               "$repoDir\.env"
 scp "${server}:/config/caddy/Caddyfile"                "$repoDir\Caddyfile"
 
 Write-Host "Done. Files synced:"