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

server/Containerfile

@@ -25,6 +25,7 @@ COPY server/src/WebDrop.Core/WebDrop.Core.csproj ./server/src/WebDrop.Core/
 COPY server/src/WebDrop.FileProtocol/WebDrop.FileProtocol.csproj ./server/src/WebDrop.FileProtocol/
 COPY server/src/WebDrop.Frontend/WebDrop.Frontend.csproj ./server/src/WebDrop.Frontend/
 COPY server/src/WebDrop.Storage/WebDrop.Storage.csproj ./server/src/WebDrop.Storage/
+COPY server/src/WebDrop.Sync/WebDrop.Sync.csproj ./server/src/WebDrop.Sync/
 RUN dotnet restore server/src/WebDrop.Api/WebDrop.Api.csproj \
     --runtime linux-x64
 COPY server/ ./server/