(config) Harden Docker deployment for remote servers

Run the Django control panel with Gunicorn instead of the development
runserver and serve static files through WhiteNoise.

Add restart policies, healthchecks, .env-driven production settings, and
a sample .env file for single-server deployments. Update the Docker
entrypoint to collect static assets and document the remote server
deployment and update flow in the README.

src/pobsync_server/settings.py

@@ -24,6 +24,7 @@ INSTALLED_APPS = [
 
 MIDDLEWARE = [
     "django.middleware.security.SecurityMiddleware",
+    "whitenoise.middleware.WhiteNoiseMiddleware",
     "django.contrib.sessions.middleware.SessionMiddleware",
     "django.middleware.common.CommonMiddleware",
     "django.middleware.csrf.CsrfViewMiddleware",
@@ -85,6 +86,11 @@ USE_TZ = True
 
 STATIC_URL = "static/"
 STATIC_ROOT = os.getenv("POBSYNC_STATIC_ROOT", str(BASE_DIR / "var" / "static"))
+STORAGES = {
+    "staticfiles": {
+        "BACKEND": "whitenoise.storage.CompressedManifestStaticFilesStorage",
+    },
+}
 
 DEFAULT_AUTO_FIELD = "django.db.models.BigAutoField"