fix: switch to node:22-slim, install deps from npm, remove vendor

infra/docker/Dockerfile.backend

@@ -1,4 +1,4 @@
-FROM node:22-alpine
+FROM node:22-slim
 
 WORKDIR /app
 
@@ -6,11 +6,10 @@ WORKDIR /app
 RUN npm install -g pnpm
 
 # Install build tools for native dependencies (node-crc, @discordjs/opus)
-# @discordjs/voice is a shared workspace dependency needed even by backend
-RUN apk add --no-cache python3 make g++
+RUN apt-get update -qq && apt-get install -y -qq --no-install-recommends python3 make g++ wget && rm -rf /var/lib/apt/lists/*
 
 # Create non-root user
-RUN addgroup -S app && adduser -S -G app app
+RUN groupadd --system app && useradd --system -g app app
 
 # Copy dependency definition files first for better caching
 COPY pnpm-workspace.yaml .