test: add deployment test tiers (shellcheck, hadolint, Docker smoke, integration)

Tier 1 — quality.yml: ShellCheck on lxc-install.sh, hadolint on both Dockerfiles
Tier 2 — docker-ci.yml: build images, smoke-test backend health + frontend 200
Tier 3 — test_integration.py: full stack pytest (auth, canvas save/reload, dimensions)
Also adds Docker healthcheck to backend service in docker-compose.yml

.github/workflows/quality.yml

@@ -7,6 +7,23 @@ on:
     branches: [main]
 
 jobs:
+  lint-scripts:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: ShellCheck — lxc-install.sh
+        uses: ludeeus/action-shellcheck@2.0.0
+        with:
+          scandir: './scripts'
+      - name: Hadolint — Dockerfile.backend
+        uses: hadolint/hadolint-action@v3.1.0
+        with:
+          dockerfile: Dockerfile.backend
+      - name: Hadolint — Dockerfile.frontend
+        uses: hadolint/hadolint-action@v3.1.0
+        with:
+          dockerfile: Dockerfile.frontend
+
   frontend:
     runs-on: ubuntu-latest
     defaults: