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/config.go

@@ -0,0 +1,36 @@
+package lobby
+
+import (
+	"fmt"
+	"time"
+)
+
+// Config configures the matchmaking pool's robot substitution.
+type Config struct {
+	// RobotWait is how long an auto-match player waits for a human before a robot
+	// is substituted. Sourced from BACKEND_LOBBY_ROBOT_WAIT.
+	RobotWait time.Duration
+	// ReaperInterval is how often the substitution reaper scans for over-waited
+	// players. Sourced from BACKEND_LOBBY_REAPER_INTERVAL.
+	ReaperInterval time.Duration
+}
+
+// DefaultConfig returns the matchmaking defaults: a 10-second wait
+// (docs/ARCHITECTURE.md §7) scanned every second.
+func DefaultConfig() Config {
+	return Config{
+		RobotWait:      10 * time.Second,
+		ReaperInterval: time.Second,
+	}
+}
+
+// Validate reports whether the configuration is usable.
+func (c Config) Validate() error {
+	if c.RobotWait <= 0 {
+		return fmt.Errorf("lobby: robot wait must be positive, got %s", c.RobotWait)
+	}
+	if c.ReaperInterval <= 0 {
+		return fmt.Errorf("lobby: reaper interval must be positive, got %s", c.ReaperInterval)
+	}
+	return nil
+}