chore: centralize app version and add CHANGELOG

[gordonmurray]

Summary

• Add VERSION at repo root as the single source of truth for the app version.
• backend/app.py reads VERSION at import; hardcoded 0.1.0 strings removed.
• Dockerfile uses an APP_VERSION build-arg for the org.opencontainers.image.version label.
• CI reads VERSION once per run and passes it as a build-arg on every matrix build.
• New :v{version} GHCR tag on the canonical Lance variant when a v* tag is pushed.
• backend/pyproject.toml version synced to 0.2.0.
• CHANGELOG.md seeded with entries since v0.1.0 in Keep-a-Changelog format.
• image.source label corrected to https://github.com/lance-format/lance-data-viewer.

After merge

1. Tag v0.2.0 on main. CI produces:
   • ghcr.io/lance-format/lance-data-viewer:stable
   • ghcr.io/lance-format/lance-data-viewer:v0.2.0
   • ghcr.io/lance-format/lance-data-viewer:app-0.2.0_lancedb-{0.3.1,0.3.4,0.5,0.16.0,0.24.3,0.29.2}
2. Create a GitHub Release from the tag with gh release create v0.2.0 --generate-notes.

Test plan

• CI build passes for all six Lance matrix entries.
• /healthz on a built image reports "app_version": "0.2.0" and "build_tag": "app-0.2.0_lancedb-0.29.2".
• Image label org.opencontainers.image.version reads 0.2.0 (docker inspect).
• On a v0.2.0 tag push, the GHCR tag list gains :stable and :v0.2.0 alongside the existing :lancedb-* entries.

Fixes #46