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

docker-compose.yml

@@ -17,6 +17,12 @@ services:
     # Required for ping-based status checks
     cap_add:
       - NET_RAW
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:8000/api/v1/health"]
+      interval: 10s
+      timeout: 5s
+      retries: 6
+      start_period: 15s
 
   mcp:
     build: