chore(docker): refine compose for named volume server deploy

Use a named SQLite volume, healthcheck, and clearer production env examples.

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.docker.example

@@ -1,5 +1,7 @@
-# Copy to .env.docker for docker compose (gitignored).
-# Do not commit real secrets.
-
-JWT_SECRET=replace-with-a-strong-secret
-CORS_ORIGIN=http://localhost:5174
+# Copy to .env.docker on the server (never commit real secrets).
+#
+# Required in production:
+JWT_SECRET=replace-with-a-long-random-secret
+#
+# Frontend origin (your deployed Vue app URL):
+CORS_ORIGIN=https://your-frontend-domain.example