[borodino] fix(rag): sentence-transformers via dedicated glibc image

Commit `08a89d3`

sentence-transformers/torch/onnxruntime cannot run on Alpine/musl (libgomp lacks pthread_attr_setaffinity_np, no musllinux wheels for onnxruntime).

Add Dockerfile.campaign-dispatcher (python:3.12-slim-bookworm/glibc): psycopg2-binary + sentence-transformers + faiss-cpu + MiniLM pre-cached
Revert Dockerfile.borodino: remove ML deps (borodino stays Alpine/lean)
Update stack: campaign-dispatcher uses campaign-dispatcher:latest image
rag_context.py: restored to sentence_transformers (works on glibc image)

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com