Adds a 'migrator' Dockerfile stage + Compose service that runs 'dotnet ef
database update' once on bring-up, so a fresh SQL volume gets the schema with no
operator step (quirk 1). cluster-seed + every host node depend on it via
service_completed_successfully, so the seed never races an in-progress migration
(quirk 2). Host build pinned to target: runtime (the migrator is now the last
stage). entrypoint + README updated; the manual 'dotnet ef' first-time step is
gone. Verified: down -v + up --build self-bootstraps (migrator+seed exit 0,
6 nodes up), deploy Sealed 6/6.
Joseph Doherty
b0a62a9f3b