Enable pip cache in Dockerfile.prod for faster builds

- Remove --no-cache-dir flag from pip install
- Use Docker layer caching for faster production builds
- Manual server rebuilds will auto-update packages if needed

backend/Dockerfile.prod

@@ -9,8 +9,8 @@ WORKDIR /app
 # Copy requirements.txt
 COPY requirements.txt .
 
-# Install dependencies to a local directory
-RUN pip install --user --no-cache-dir -r requirements.txt
+# Install dependencies to a local directory (with cache for speed)
+RUN pip install --user -r requirements.txt
 
 # ============================================================
 # Final stage - Production image (tối giản)