Fix Dockerfile: add build-essential for numcodecs compilation

numcodecs requires C compiler during pip install.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Dockerfile

@@ -1,9 +1,10 @@
 # Backend-only Dockerfile for Hugging Face Spaces
 FROM python:3.11-slim
 
-# Install system dependencies
+# Install system dependencies (including build tools for numcodecs)
 RUN apt-get update && apt-get install -y \
     curl \
+    build-essential \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app