R1: schema & naming reset — squash migrations, rename variants

Squash the 12 goose migrations into one 00001_baseline.sql (there is no prod
data; verified schema-identical to the chain via a pg_dump diff + the green
integration suite) and rename the game-variant labels
english/russian_scrabble/erudit -> scrabble_en/scrabble_ru/erudit_ru across the
backend, the FlatBuffers wire values and the UI.

dawg filenames and the Go enum identifiers are unchanged; the i18n display keys
are kept. Adds PRERELEASE.md (the R1-R7 pre-release tracker), linked from
CLAUDE.md. Contour DB wipe and the scrabble-dictionary tidy are follow-ups.

backend/internal/postgres/migrate.go

@@ -37,7 +37,7 @@ var gooseMu sync.Mutex
 // ApplyMigrations runs every pending Up migration embedded in the backend
 // binary against db. The schema is created upfront so goose's bookkeeping table
 // (`goose_db_version`, scoped to the DSN search_path) has somewhere to land
-// before the first migration runs; migration 00001_init.sql re-asserts the
+// before the first migration runs; the baseline migration re-asserts the
 // schema with IF NOT EXISTS, so the double-create is idempotent.
 //
 // The apply is retried on transient connection errors. Both steps are