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.

backend/internal/lobby/lobby.go

@@ -21,6 +21,13 @@ type GameCreator interface {
 	Create(ctx context.Context, params game.CreateParams) (game.Game, error)
 }
 
+// RobotProvider supplies a robot account to substitute for a missing human in
+// auto-match. robot.Service satisfies it; it returns an error when no robot is
+// available so the matchmaker can defer substitution.
+type RobotProvider interface {
+	Pick() (uuid.UUID, error)
+}
+
 // Blocker reports whether two accounts have a block between them (either
 // direction). social.Service satisfies it; the lobby uses it to refuse
 // invitations between blocked accounts.