chore(docker): stack compose with env_file, no secrets in repo

- docker-compose.stack.env for DB_* and CORS (gitignored)
- docker-compose.stack.env.example with placeholders
- .dockerignore excludes local stack env from build context

Made-with: Cursor

Dockerfile.backend

@@ -0,0 +1,22 @@
+# Сборка из корня монорепо: docker build -f Dockerfile.backend .
+FROM node:20-alpine AS deps
+WORKDIR /app
+COPY backend/package.json backend/package-lock.json ./
+RUN npm ci
+
+FROM deps AS build
+COPY backend/tsconfig.json ./
+COPY backend/src ./src
+RUN npm run build
+
+FROM node:20-alpine AS runner
+WORKDIR /app
+ENV NODE_ENV=production
+COPY backend/package.json backend/package-lock.json ./
+RUN npm ci --omit=dev
+COPY --from=build /app/dist ./dist
+COPY backend/migrations ./migrations
+COPY import ./import
+EXPOSE 3000
+ENV PORT=3000
+CMD ["node", "dist/index.js"]