fix(docker): Actually use CACHE_BUST ARG to invalidate cache

Added RUN command that references CACHE_BUST ARG to force Docker to
invalidate cache and rebuild subsequent layers.

Issue: ARG was declared but never used, so Docker continued using
cached layers with CUDA PyTorch.

Fix: Added 'RUN echo "Cache bust: $CACHE_BUST"' which forces Docker
to execute this layer whenever CACHE_BUST value changes, invalidating
all subsequent cached layers including poetry install.

Related: #506

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: manavgup

backend/Dockerfile.backend

@@ -33,6 +33,7 @@ WORKDIR /app
 # Previous: POETRY_ROOT_MIGRATION=20251027 (Poetry root migration)
 # Updated: 20251028 to invalidate cache and install CPU-only PyTorch
 ARG CACHE_BUST=20251028
+RUN echo "Cache bust: $CACHE_BUST"
 
 # Copy dependency files first for better layer caching
 COPY pyproject.toml poetry.lock ./