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/account/validate_test.go

@@ -24,7 +24,16 @@ func TestValidateDisplayName(t *testing.T) {
 		"trailing underscore": {"Name_", "", false},
 		"trailing dot ok":     {"Anna B.", "Anna B.", true},
 		"double trailing dot": {"Name..", "", false},
-		"digit rejected":      {"Name2", "", false},
+		"trailing digit ok":   {"Name2", "Name2", true},
+		"trailing year ok":    {"Аня2007", "Аня2007", true},
+		"five digits ok":      {"Player12345", "Player12345", true},
+		"six digits rejected": {"Player123456", "", false},
+		"mid digit rejected":  {"Dark2Wolf", "", false},
+		"all digits rejected": {"12345", "", false},
+		"digit then dot":      {"Name2.", "", false},
+		"dot then digit":      {"Anna B.2", "", false},
+		"sep plus digits ok":  {"Night.Fox2007", "Night.Fox2007", true},     // "." is the only special; digits do not count
+		"max specials+digits": {"a.a.a.a.a.a2007", "a.a.a.a.a.a2007", true}, // 5 dots + a digit run still passes
 		"blank":               {"   ", "", false},
 		"too long":            {strings.Repeat("a", 33), "", false},
 		"five specials ok":    {"a.a.a.a.a.a", "a.a.a.a.a.a", true},           // 5 dots