feat(docker): add multi-stage image and compose with persistent SQLite

Mount ./docker-data for WAL-capable SQLite storage, support DATABASE_PATH alias, and document compose secrets via .env.docker.example.

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

.env.example

@@ -6,8 +6,9 @@ NODE_ENV=development
 # CORS (Vue dev server)
 CORS_ORIGIN=http://localhost:5173
 
-# SQLite — production database
-# DB_PATH=E:/absolute/path/to/backend-nest/data/app.db
+# SQLite — path inside container or on host (DB_PATH or DATABASE_PATH)
+# DB_PATH=/app/data/app.db
+# DATABASE_PATH=/app/data/app.db
 
 # JWT — must match legacy Fastify when migrating existing tokens
 JWT_SECRET=gold-land-dev-secret-change-in-production