feat(robot): per-game display names from a wide name corpus

Decouple the displayed opponent name from the small pool of durable robot
accounts: the disguised auto-match robot now gets a freshly composed name each
game, stamped on a new game_players.display_name seat snapshot. The snapshot
also captures humans' names, freezing what an opponent sees for the life of a
game (a later rename no longer rewrites past games); readers fall back to the
account's current name for pre-migration rows.

Names come from a wide composed corpus (internal/robot/namevariety.go): Western
locales (EN/DE/ES/IT/FR/PT), native Japanese/Chinese names, a gender-agreed
Russian pool, and human-style handles. Routing keeps Pick's spirit -- a Russian
game draws Cyrillic + <=20% Latin and never a CJK script; an English game the
full corpus -- via robot.PickNamed.

Loosen account.ValidateDisplayName (and the UI mirror) to admit a trailing run
of up to five digits, so "Player2007"-style handles are valid for humans too
and the disguised robot stays indistinguishable.

Migration 00007 adds game_players.display_name (additive, NOT NULL DEFAULT '');
jet regenerated. Docs (ARCHITECTURE 7, FUNCTIONAL + _ru, PLAN, README) updated.

backend/internal/postgres/migrations/00007_seat_display_name.sql

@@ -0,0 +1,16 @@
+-- +goose Up
+-- A per-seat snapshot of the player's display name, captured when the seat is taken:
+-- the human's then-current display name, or a disguised robot's freshly composed
+-- per-game name. Storing the name on the seat (rather than always reading the account)
+-- freezes what the opponent sees for the life of the game — a later rename no longer
+-- rewrites past games — and lets the small robot pool present an ever-changing crowd of
+-- differently named opponents. An empty value means "no snapshot": the reader falls back
+-- to the account's current display name (legacy rows / pre-migration games). See
+-- docs/ARCHITECTURE.md §7.
+SET search_path = backend, pg_catalog;
+
+ALTER TABLE game_players ADD COLUMN display_name text NOT NULL DEFAULT '';
+
+-- +goose Down
+SET search_path = backend, pg_catalog;
+ALTER TABLE game_players DROP COLUMN display_name;