Stage 5: robot opponent (pool, seed-derived strategy, move driver, matchmaker substitution)

- internal/robot: durable kind='robot' account pool (migration 00004); every
  per-game and per-turn choice derived deterministically from the game seed
  (restart-stable FNV mix); a background move driver; margin targeting (band
  1-30, closest-to-band); right-skewed [2,90]min delays (median ~10m);
  opponent-anchored sleep with +/-3h drift; daytime nudge reply + proactive
  12h nudge; friend/chat blocked via profile toggles.
- engine.Candidates (decoded ranked plays); game.Candidates + RobotTurns;
  social.LastNudgeAt.
- matchmaker: 10s wait then robot substitution (reaper) + Poll delivery seam.
- config (BACKEND_ROBOT_DRIVE_INTERVAL, BACKEND_LOBBY_ROBOT_WAIT,
  BACKEND_LOBBY_REAPER_INTERVAL); main wiring + boot-time pool provisioning.
- metrics: robot account_stats (authoritative balance) + robot_games_finished_total
  OTel counter + per-finish log.
- docs: PLAN, ARCHITECTURE, FUNCTIONAL(+ru), TESTING, README; account.go comment.
- tests: robot strategy units, matchmaker reaper/Poll, engine.Candidates; inttest
  robot full-game / substitution / proactive-nudge.

docs/FUNCTIONAL.md

@@ -49,9 +49,14 @@ the bag or removed from play) is chosen when the game is created, and the leaver
 rack is never shown to the others.
 
 ### Robot opponent *(Stage 5)*
-Indistinguishable-from-human substitute in auto-match. Decides once whether to
-play to win (~40%), targets a small score margin, plays with human-like timing
-and a night sleep window, and nudges/answers nudges like a person.
+When auto-match finds no human within ten seconds, a robot opponent takes the empty
+seat so the game starts without waiting. It is meant to feel like a person: it
+decides once per game whether to play to win (about 40% of the time, so the human
+wins most games), aims for a close score rather than crushing or throwing the game,
+and plays at a human pace — short thinking times for most moves, the occasional long
+one, and a night-time pause that tracks the player's own day. It answers a nudge
+within a few minutes and nudges back when the player has been away a long time. It
+carries a human-like name and neither chats nor accepts friend requests.
 
 ### Social: friends, block, chat, nudge *(Stage 4)*
 Send a friend request and have it accepted (decline or cancel withdraws it,