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/lobby/matchmaker.go

@@ -19,7 +19,7 @@ import (
 // it.
 type GameMatcher interface {
 	OpenOrJoin(ctx context.Context, accountID uuid.UUID, params game.CreateParams, openDeadline time.Time) (game.Game, bool, error)
-	AttachRobot(ctx context.Context, gameID, robotID uuid.UUID) (game.Game, bool, error)
+	AttachRobot(ctx context.Context, gameID, robotID uuid.UUID, displayName string) (game.Game, bool, error)
 	ExpiredOpen(ctx context.Context, now time.Time) ([]game.OpenGame, error)
 	InitialState(ctx context.Context, gameID, accountID uuid.UUID) (notify.PlayerState, error)
 	// Create seats the given accounts in an active game at once; the AI-match path uses
@@ -148,12 +148,12 @@ func (m *Matchmaker) Reap(ctx context.Context, now time.Time) {
 		return
 	}
 	for _, og := range due {
-		robotID, err := m.robots.Pick(og.Variant)
+		robotID, name, err := m.robots.PickNamed(og.Variant)
 		if err != nil {
 			m.log.Warn("robot substitution deferred", zap.Error(err))
 			continue
 		}
-		g, attached, err := m.games.AttachRobot(ctx, og.ID, robotID)
+		g, attached, err := m.games.AttachRobot(ctx, og.ID, robotID, name)
 		if err != nil {
 			m.log.Warn("robot substitution failed", zap.String("game", og.ID.String()), zap.Error(err))
 			continue