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/README.md

@@ -47,7 +47,12 @@ services are exposed via `Server` accessors for those handlers.
 
 The robot opponent (`internal/robot`). A pool of durable accounts —
 each a `kind='robot'` identity, provisioned at startup with chat and friend
-requests blocked — backs human-like, per-language composed names. A background driver plays the
+requests blocked — carries human-like names. The disguised reaper stamps a **fresh per-game name** on
+the robot's seat (a `game_players.display_name` snapshot — which also freezes humans' names per game, so
+a later rename never rewrites past games) drawn from a wide composed corpus (Western locales, native
+Japanese/Chinese, a gender-agreed Russian pool, and handles); a Russian game stays Cyrillic with ≤20%
+Latin and no CJK script, an English game uses the full corpus (`namevariety.go`, `PickNamed`). A
+background driver plays the
 robot's moves through the public game API as an ordinary seated player (so only
 `internal/engine` imports the solver): it decides once per game whether to play to
 win (≈ 40%), targets a small score margin — with an occasional off-strategy move that tapers to