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/lobby.go

@@ -33,6 +33,9 @@ type GameCreator interface {
 // available so the matchmaker can defer substitution.
 type RobotProvider interface {
 	Pick(variant engine.Variant) (uuid.UUID, error)
+	// PickNamed selects a robot and composes a fresh per-game display name for it, for
+	// the disguised auto-match path (the name is stamped on the robot's seat).
+	PickNamed(variant engine.Variant) (uuid.UUID, string, error)
 }
 
 // Blocker reports whether two accounts have a block between them (either