Fix backend Dockerfile - remove unnecessary build step

The backend is designed to run TypeScript directly with Bun, not to build.
Updated Dockerfile to skip build step and run src/index.ts directly.

backend/Dockerfile

@@ -1,28 +1,20 @@
 FROM node:20-alpine AS base
 WORKDIR /app
 
-# Install dependencies
+# Install Bun
-FROM base AS install
 RUN npm install -g bun
-COPY package.json ./
+
+# Install dependencies
+COPY package.json bun.lockb* ./
 RUN bun install
 
-# Build
+# Copy source code
-FROM base AS build
-COPY --from=install /app/node_modules ./node_modules
 COPY . .
-RUN bun build
-
-# Production
-FROM base AS release
-RUN npm install -g bun
-COPY --from=install /app/node_modules ./node_modules
-COPY --from=build /app/dist ./dist
-COPY package.json .
 
 ENV NODE_ENV=production
 ENV PORT=3001
 
 EXPOSE 3001
 
-CMD ["bun", "run", "start"]
+# Run directly from TypeScript source (Bun can execute TypeScript directly)
+CMD ["bun", "run", "src/index.ts"]