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/account/account.go

@@ -21,10 +21,12 @@ import (
 
 // Identity kinds recognised by the backend. Email is modelled as an identity
 // alongside platform identities; its confirmed flag is driven by the email
-// confirm-code flow in a later stage.
+// confirm-code flow in a later stage. Robot is a synthetic kind: each pooled
+// robot opponent is a durable account bound to one robot identity (Stage 5).
 const (
 	KindTelegram = "telegram"
 	KindEmail    = "email"
+	KindRobot    = "robot"
 )
 
 // uniqueViolation is the PostgreSQL SQLSTATE for a unique-constraint violation.
@@ -34,10 +36,12 @@ const uniqueViolation = "23505"
 var ErrNotFound = errors.New("account: not found")
 
 // Account is a durable internal account. AwayStart and AwayEnd bound the daily
-// local-time window (in TimeZone) during which the player is asleep: the
-// turn-timeout sweeper does not auto-resign them inside it, and the robot reuses
-// it for its own sleep in a later stage. HintBalance is the player's wallet of
-// purchasable hints, spent after a game's per-seat allowance.
+// local-time window (in TimeZone) during which the player is asleep, so the
+// turn-timeout sweeper does not auto-resign them inside it. (The robot opponent's
+// own sleep is anchored to its human opponent's timezone with a per-game drift,
+// computed in internal/robot, not from a robot account's away window.) HintBalance
+// is the player's wallet of purchasable hints, spent after a game's per-seat
+// allowance.
 type Account struct {
 	ID                  uuid.UUID
 	DisplayName         string