feat: unify on single repo-root compose file for dev and prod

.forgejo/workflows/infra.yml

@@ -0,0 +1,22 @@
+# Validates the single compose file parses (needs only the CLI, no daemon).
+# Same Runner prerequisites as backend.yml.
+on:
+  push:
+    branches: [main]
+  pull_request:
+    paths:
+      - "docker-compose.yml"
+      - ".env.example"
+      - "backend/Dockerfile"
+      - "frontend/Dockerfile"
+      - "Dockerfile.lfx"
+      - ".forgejo/workflows/infra.yml"
+
+jobs:
+  validate:
+    runs-on: docker
+    container:
+      image: docker:24
+    steps:
+      - uses: actions/checkout@v4
+      - run: docker compose -f docker-compose.yml config --quiet