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/adminconsole/templates/pages/dictionary.gohtml

@@ -30,9 +30,9 @@
 <form class="form" method="post" action="/_gm/dictionary/changes/apply">
 <label>Mark applied for variant
 <select name="variant">
-<option value="english">english</option>
-<option value="russian_scrabble">russian_scrabble</option>
-<option value="erudit">erudit</option>
+<option value="scrabble_en">scrabble_en</option>
+<option value="scrabble_ru">scrabble_ru</option>
+<option value="erudit_ru">erudit_ru</option>
 </select>
 </label>
 <label>In version <input type="text" name="version" placeholder="v2" required></label>