build: optimize Dockerfile for production and improve .dockerignore

.dockerignore

@@ -1,5 +1,7 @@
 node_modules
+**/node_modules
 .git
+.gitignore
 .env
 db
 docs

Dockerfile

@@ -7,14 +7,18 @@ RUN apk add --no-cache ffmpeg chromaprint
 
 # Install dependencies
 COPY server/package*.json ./server/
-RUN cd server && npm install --production
+RUN cd server && npm ci --omit=dev
 
 # Copy app source
 COPY server/ ./server/
 COPY public/ ./public/
 COPY shared/ ./shared/
 
+# Set ownership and switch to non-root user
+RUN chown -R node:node /app
+USER node
+
 # Start server
 WORKDIR /app/server
 EXPOSE 3000
-CMD ["npm", "start"]
+CMD ["node", "index.js"]