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

apply-compose.ps1

@@ -2,7 +2,7 @@
 # to the live Docker stack.
 #
 # On valhalla, `dc` is aliased (in ~/.bashrc) to:
-#   docker-compose -f ~/htpc-download-box/docker-compose.yml --env-file ~/htpc-download-box/.env
+#   docker compose -f ~/valhalla-lab/docker-compose.yml --env-file ~/valhalla-lab/.env
 # It's a shell alias, so we invoke it over SSH with `bash -ic` to load it.
 #
 # Usage (flags combine; no flags = push & apply all three):
@@ -33,7 +33,7 @@ if (-not $Compose -and -not $Caddy -and -not $EnvFile) {
 
 if ($EnvFile) {
     Write-Host "Pushing .env ..."
-    scp $envLocal "${server}:~/htpc-download-box/.env"
+    scp $envLocal "${server}:~/valhalla-lab/.env"
 }
 
 if ($Caddy) {
@@ -45,7 +45,7 @@ if ($Caddy) {
 
 if ($Compose) {
     Write-Host "Pushing docker-compose.yml ..."
-    scp $composeLocal "${server}:~/htpc-download-box/docker-compose.yml"
+    scp $composeLocal "${server}:~/valhalla-lab/docker-compose.yml"
     Write-Host "Pulling latest images ..."
     ssh $server "bash -ic 'dc pull'"
     Write-Host "Applying compose changes (recreates only changed containers) ..."