ci: build and release webdrop-api and webdrop-files as separate images [CI]

- dev images are built only when the push touched code that image uses (path filters);
  webdrop-files does not wait for the frontend build
- one v* tag releases both images (:tag and :latest) after the shared checks
- registry build and push live in scripts/ci-publish-image.sh, used by every workflow
- containers: fix the api Containerfile (missing Sync project), add Containerfile.files,
  Quadlet units for the file service (prod and dev), env examples, and registry
  auto-update for the production units so each machine pulls only the image it runs

deploy/webdrop-dev.container

@@ -10,6 +10,7 @@ PublishPort=127.0.0.1:5001:5000
 Environment=ASPNETCORE_URLS=http://+:5000
 Environment=WebDrop__DataDir=/data
 Environment=DOTNET_EnableDiagnostics=0
+EnvironmentFile=-%h/podman/webdrop-dev/webdrop.env
 Volume=%h/podman/webdrop-dev/data:/data:Z,U
 ReadOnly=true
 ReadOnlyTmpfs=false