Extended Server.Release with migrate/0 and rollback/2 so
'bin/server eval Server.Release.migrate' works from a released binary.
Removed the phx.gen.release-generated rel/overlays/bin/server wrapper
that hardcoded 'start' — it collided with the mix-release default
dispatcher, blocking 'server version', 'server eval', etc. The 'migrate'
overlay is kept (bin/migrate calls server eval under the hood).
Blocking bootstrap in dev meant you couldn't even run 'mix run' to
generate the initial hash. Now dev/test accept an optional env override
and boot without it; prod still raises when unset.
Application.start ran mark_all_offline unconditionally, which meant
every "mix run"/"mix ecto.migrate" invocation would flip all
connected hosts to offline. Gate the call on Phoenix.Endpoint.server?
so non-serving boots don't disturb live state.
