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.

docs/ARCHITECTURE.md

@@ -556,7 +556,7 @@ promotions) is future work and would deliver short markdown messages (text + lin
   `robot_games_finished_total`; an observable gauge `game_cache_active`; the gateway
   `edge_request_duration` (the UI-perceived roundtrip, by `message_type`/`result`);
   and Go runtime/heap metrics. Game-scoped metrics carry a `variant` attribute
-  (english/russian_scrabble/erudit).
+  (scrabble_en/scrabble_ru/erudit_ru).
 - Per-user move-time analytics (Stage 17) are **offline**, derived in the admin
   console from the move journal (`game_moves.created_at` deltas, the first move from
   the game's creation), not Prometheus labels (which an `account_id` would explode):