chore(mix release): Ship Docker release image and release-safe Typesense migration

[ThaddeusJiang]

Summary

• switch the Docker image to a multi-stage Elixir release build with a slimmer runtime image
• add a release-safe SaveIt.Release.ts_migrate/0 entrypoint and reuse the same migration logic from mix ts.migrate
• document the new release command and record the Mix.env() runtime crash postmortem

Testing

• mise trust
• mise install
• MIX_ENV=prod mix release --overwrite
• docker build -t save_it:ac .
• docker run --rm save_it:ac /app/bin/save_it eval 'IO.puts("release-ok")'
• TYPESENSE_URL=http://127.0.0.1:9 _build/prod/rel/save_it/bin/save_it eval 'SaveIt.Release.ts_migrate()'
• git diff --check