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/config/config.go

@@ -10,7 +10,9 @@ import (
 
 	"scrabble/backend/internal/account"
 	"scrabble/backend/internal/game"
+	"scrabble/backend/internal/lobby"
 	"scrabble/backend/internal/postgres"
+	"scrabble/backend/internal/robot"
 	"scrabble/backend/internal/telemetry"
 )
 
@@ -26,6 +28,10 @@ type Config struct {
 	Telemetry telemetry.Config
 	// Game configures the game subsystem (dictionaries, sweeper, live-game cache).
 	Game game.Config
+	// Lobby configures matchmaking robot substitution (wait window, reaper cadence).
+	Lobby lobby.Config
+	// Robot configures the robot opponent driver (scan cadence).
+	Robot robot.Config
 	// SMTP configures the email relay used for confirm-codes. An empty Host
 	// selects the development log mailer (the code is logged, not sent).
 	SMTP account.SMTPConfig
@@ -71,6 +77,19 @@ func Load() (Config, error) {
 		return Config{}, err
 	}
 
+	lb := lobby.DefaultConfig()
+	if lb.RobotWait, err = envDuration("BACKEND_LOBBY_ROBOT_WAIT", lb.RobotWait); err != nil {
+		return Config{}, err
+	}
+	if lb.ReaperInterval, err = envDuration("BACKEND_LOBBY_REAPER_INTERVAL", lb.ReaperInterval); err != nil {
+		return Config{}, err
+	}
+
+	rb := robot.DefaultConfig()
+	if rb.DriveInterval, err = envDuration("BACKEND_ROBOT_DRIVE_INTERVAL", rb.DriveInterval); err != nil {
+		return Config{}, err
+	}
+
 	smtp := account.SMTPConfig{
 		Host:     os.Getenv("BACKEND_SMTP_HOST"),
 		Port:     envOr("BACKEND_SMTP_PORT", "587"),
@@ -85,6 +104,8 @@ func Load() (Config, error) {
 		Postgres:  pg,
 		Telemetry: tel,
 		Game:      gm,
+		Lobby:     lb,
+		Robot:     rb,
 		SMTP:      smtp,
 	}
 	if err := c.validate(); err != nil {
@@ -112,6 +133,12 @@ func (c Config) validate() error {
 	if err := c.Game.Validate(); err != nil {
 		return fmt.Errorf("config: %w (set BACKEND_DICT_DIR)", err)
 	}
+	if err := c.Lobby.Validate(); err != nil {
+		return fmt.Errorf("config: %w", err)
+	}
+	if err := c.Robot.Validate(); err != nil {
+		return fmt.Errorf("config: %w", err)
+	}
 	return nil
 }