feat(server): phoenix release with migrate/rollback helpers

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).

server/lib/server/release.ex

@@ -1,8 +1,12 @@
 defmodule Server.Release do
-  @moduledoc "Convenience functions for IEx and future release tasks."
+  @moduledoc "Convenience functions for IEx and release-stage admin tasks."
+
+  @app :server
 
   @doc "Create a host and print the plaintext token once."
   def register_host(name) do
+    load_app!()
+
     case Server.Hosts.create_host(name) do
       {:ok, {host, token}} ->
         IO.puts("Host '#{host.name}' registered (id=#{host.id}).")
@@ -15,4 +19,25 @@ defmodule Server.Release do
         {:error, cs}
     end
   end
+
+  @doc "Run pending migrations. Invoke via: bin/server eval 'Server.Release.migrate()'"
+  def migrate do
+    load_app!()
+
+    for repo <- repos() do
+      {:ok, _, _} = Ecto.Migrator.with_repo(repo, &Ecto.Migrator.run(&1, :up, all: true))
+    end
+  end
+
+  @doc "Roll back one step per repo."
+  def rollback(repo, version) do
+    load_app!()
+    {:ok, _, _} = Ecto.Migrator.with_repo(repo, &Ecto.Migrator.run(&1, :down, to: version))
+  end
+
+  defp repos, do: Application.fetch_env!(@app, :ecto_repos)
+
+  defp load_app! do
+    Application.load(@app)
+  end
 end