Restore CI and releases for legacy LanceDB variants

[gordonmurray]

Closes #92.

What changed

• use the last published multi-arch images as frozen dependency sources for LanceDB 0.3.1, 0.3.4, and 0.5
• rebuild clean runtime images from those dependencies plus the current application source
• run legacy API tests in containerized environments derived from the same preserved dependencies
• keep clean PyPI installs unchanged for all five modern variants
• document the separate local legacy build path

Why

The three legacy LanceDB releases are no longer available from PyPI, so a clean install fails before the project tests begin. The existing GHCR images still contain the exact supported dependency sets on both amd64 and arm64.

Validation

• full local matrix: all 8 variants passed all 64 API tests
• rebuilt application images for all 3 legacy variants on amd64
• 0.3.4 image passed /healthz, /datasets, /, and /app.js smoke checks
• confirmed all 3 published dependency images contain linux/amd64 and linux/arm64 manifests