v0.5.0 — pre-built ghcr.io images + real docker-run smoke test

The "no more local build" release. Closes the structural gap that produced five hotfix cycles in a row (v0.4.1 → v0.4.5): every release shipped a runtime issue that docker build could not catch but docker run would have. Publishes pre-built multi-arch images to ghcr.io so the Supervisor pulls instead of building, and adds a real docker run smoke phase to the build CI so the same class of bug surfaces in pull requests.

Added

• .github/workflows/builder.yml — home-assistant/builder builds per-arch images (amd64, aarch64) on every published GitHub Release, pushes to ghcr.io/dibi73/{arch}-bookstack-rag-addon.
• bookstack-rag/config.yaml — image: ghcr.io/dibi73/{arch}-bookstack-rag-addon. Supervisor pulls the matching manifest instead of building locally.
• bookstack-rag/config.yaml — watchdog: http://[HOST]:[PORT:8000]/api/status for a real "ready" indicator on the add-on detail page.
• .github/workflows/build.yml — after the smoke build, docker runs the image with a synthetic /data/options.json, polls /api/status for up to 120s, fails the job if the response isn't ok/initializing.

Changed

• bookstack-rag/Dockerfile — FROM ${BUILD_FROM} again (python:3.12-slim-bookworm default for local builds); ARG BUILD_FROM moved to the very top of the file, before any FROM — a multi-stage Dockerfile ARG declared between two FROM lines is only visible in the previous stage's RUN steps, not the next FROM (Buildx: "base name should not be blank").
• READMEs/DOCS drop the "first install 5–15 min" warning, replaced with the ~2 min ghcr.io-pull expectation.

Notes

• 95 tests pass (no Python changes in this release apart from the version bump).
• First-time installers still need ~3 GB free disk (PyTorch + nomic-embed-text-v1.5) and ~2 GB free RAM at runtime — only the wall-clock time to a ready state on first install drops.
• ghcr.io packages must be public for Supervisor to pull anonymously.