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/inttest/open_match_test.go

@@ -161,11 +161,11 @@ func TestOpenGameResignRejectedUntilOpponent(t *testing.T) {
 		t.Fatalf("resign while open = %v, want ErrNoOpponentYet", err)
 	}
 
-	robotID, err := robots.Pick(engine.VariantEnglish)
+	robotID, name, err := robots.PickNamed(engine.VariantEnglish)
 	if err != nil {
 		t.Fatalf("pick: %v", err)
 	}
-	if _, attached, err := svc.AttachRobot(ctx, g.ID, robotID); err != nil || !attached {
+	if _, attached, err := svc.AttachRobot(ctx, g.ID, robotID, name); err != nil || !attached {
 		t.Fatalf("attach robot = (attached %v, err %v), want attached", attached, err)
 	}
 	res, err := svc.Resign(ctx, g.ID, starter)