fix(runtime): validate vector store selection

[JSONbored]

Summary

• reject ambiguous external vector-store env combinations before OpenMemory starts
• document the real one-backend selection model in the Unraid template and docs
• add focused coverage for bundled Qdrant, external Qdrant, partial PGVector, and competing backend selectors

What changed

• validates QDRANT_, REDIS_URL, PG_, Chroma, Weaviate, Milvus, Elasticsearch, OpenSearch, and FAISS selectors in the wrapper
• sets s6 to fail fast when cont-init validation fails
• tightens Advanced View help text in the source XML
• extends the shared container contract to require s6 fail-fast behavior

Why

• OpenMemory can only initialize one vector store, and the previous env priority could silently hide impossible combinations such as Redis plus PGVector plus external Qdrant

Validation

• .venv-local/bin/pytest tests/unit tests/template
• .venv-local/bin/python scripts/validate-template.py
• .venv-local/bin/pytest tests/integration/test_container_runtime.py
• manual invalid-container probe with REDIS_URL + PG_* + QDRANT_URL returned exit status 1
• trunk check on touched files
• git diff --check

Notes

• awesome-unraid is intentionally untouched; catalog templates are synced from source AIO repos.