feat: add ktx2 texture fallback flow

Dockerfile

@@ -24,15 +24,26 @@ RUN npm run build
 # --- Stage 3: Production -----------------------------------------------------
 FROM node:20-slim AS runner
 
+ARG KTX_SOFTWARE_VERSION=4.4.2
+
 LABEL maintainer="La Fabrik Durable"
-LABEL description="Secure GLTF upload interface with Draco compression and Git push"
+LABEL description="Secure GLTF upload interface with Draco/KTX2 compression and Git push"
 
 # Blender is required for server-side Draco GLB export.
+# KTX-Software provides toktx for optional KTX2 texture delivery.
 RUN apt-get update && apt-get install -y --no-install-recommends \
+    bzip2 \
     blender \
+    ca-certificates \
     tini \
     curl \
-    && rm -rf /var/lib/apt/lists/*
+    && curl -fsSL \
+      "https://github.com/KhronosGroup/KTX-Software/releases/download/v${KTX_SOFTWARE_VERSION}/KTX-Software-${KTX_SOFTWARE_VERSION}-Linux-x86_64.tar.bz2" \
+      -o /tmp/ktx-software.tar.bz2 \
+    && mkdir -p /opt/ktx-software \
+    && tar -xjf /tmp/ktx-software.tar.bz2 -C /opt/ktx-software --strip-components=1 \
+    && ln -s /opt/ktx-software/bin/toktx /usr/local/bin/toktx \
+    && rm -rf /tmp/ktx-software.tar.bz2 /var/lib/apt/lists/*
 
 WORKDIR /app