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_test.go

@@ -45,7 +45,7 @@ func (s *stubMatcher) OpenOrJoin(_ context.Context, _ uuid.UUID, _ game.CreatePa
 	return s.openGame, s.openJoined, s.openErr
 }
 
-func (s *stubMatcher) AttachRobot(_ context.Context, gameID, _ uuid.UUID) (game.Game, bool, error) {
+func (s *stubMatcher) AttachRobot(_ context.Context, gameID, _ uuid.UUID, _ string) (game.Game, bool, error) {
 	if s.attachErr != nil {
 		return game.Game{}, false, s.attachErr
 	}
@@ -85,6 +85,11 @@ func (f *fakeRobots) Pick(variant engine.Variant) (uuid.UUID, error) {
 	return f.id, nil
 }
 
+func (f *fakeRobots) PickNamed(variant engine.Variant) (uuid.UUID, string, error) {
+	id, err := f.Pick(variant)
+	return id, "Robot", err
+}
+
 // capturePub records every published intent.
 type capturePub struct{ intents []notify.Intent }