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/robot/robot.go

@@ -175,6 +175,21 @@ func (s *Service) Pick(variant engine.Variant) (uuid.UUID, error) {
 	return primary[rand.IntN(len(primary))], nil
 }
 
+// PickNamed selects a robot account like Pick and, alongside it, composes a fresh,
+// variant-appropriate per-game display name (composeRandomName) for the matchmaker to
+// stamp on the robot's seat. Unlike the account's seeded fallback name, this name is
+// generated anew for every match, so the small pool of durable accounts presents as an
+// ever-changing crowd of opponents; it stays within account.ValidateDisplayName's
+// format so a disguised robot remains indistinguishable from a human
+// (docs/ARCHITECTURE.md §7).
+func (s *Service) PickNamed(variant engine.Variant) (uuid.UUID, string, error) {
+	id, err := s.Pick(variant)
+	if err != nil {
+		return uuid.Nil, "", err
+	}
+	return id, composeRandomName(variant), nil
+}
+
 // poolIDs returns a snapshot of the whole pool (both languages) for the driver scan,
 // which is variant-agnostic — it acts on every robot's active games.
 func (s *Service) poolIDs() []uuid.UUID {