Add system and Python dependencies for chart generation (matplotlib fix)

Dockerfile

@@ -11,10 +11,9 @@ RUN apt-get update && apt-get install -y \
     pkg-config \
     libfreetype6-dev \
     libpng-dev \
-    g++ \
+    build-essential \
     # --- End FIX ---
     && rm -rf /var/lib/apt/lists/*
-
 # 2. Create non-root user
 RUN useradd -m appuser
 # 3. Create directory structure (with proper ownership from the start)