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/README.md

@@ -44,10 +44,22 @@ remains. As before this is a service/store layer — chat and nudges are persist
 but their live delivery, and all REST endpoints, arrive with the `gateway`
 (Stage 6); the services are exposed via `Server` accessors for those handlers.
 
+Stage 5 adds the robot opponent (`internal/robot`). A pool of durable accounts —
+each a `kind='robot'` identity, provisioned at startup with chat and friend
+requests blocked — backs a human-like name pool. A background driver plays the
+robot's moves through the public game API as an ordinary seated player (so only
+`internal/engine` imports the solver): it decides once per game whether to play to
+win (≈ 40%), targets a small score margin, and times its moves with a right-skewed
+delay, a night-sleep window anchored to the opponent's timezone, and nudge
+behaviour — all derived deterministically from the game seed, so it keeps no extra
+state. The matchmaker now substitutes a pooled robot after a 10-second wait and
+exposes `Poll` so a waiting player can collect the started game (the live
+match-found notification arrives with the `gateway`).
+
 ## Package layout
 
 ```
-cmd/backend/         # entrypoint: telemetry -> db+migrate -> registry -> cache -> game+sweeper -> lobby+social -> server
+cmd/backend/         # entrypoint: telemetry -> db+migrate -> registry -> cache -> game+sweeper -> robot pool+driver -> lobby+social -> server
 cmd/jetgen/          # dev tool: regenerate go-jet code from a throwaway container
 internal/config/     # env configuration (composes postgres + telemetry + game config)
 internal/telemetry/  # OpenTelemetry providers + per-request timing middleware
@@ -60,7 +72,8 @@ internal/server/     # gin engine, route groups, X-User-ID middleware, probes
 internal/engine/     # in-process scrabble-solver bridge: registry, bag, Game, replay
 internal/game/       # game domain: lifecycle, journal+cache, hint, word-check, GCG, sweeper
 internal/social/     # friend graph, per-user blocks, per-game chat + nudge, content filter
-internal/lobby/      # in-memory matchmaking pool + friend-game invitations
+internal/lobby/      # in-memory matchmaking pool (+ robot substitution) + friend-game invitations
+internal/robot/      # human-like robot opponent: account pool, seed-derived strategy, move driver
 ```
 
 ## Configuration (environment)
@@ -81,6 +94,9 @@ internal/lobby/      # in-memory matchmaking pool + friend-game invitations
 | `BACKEND_DICT_VERSION` | `v1` | Dictionary version new games pin. |
 | `BACKEND_GAME_TIMEOUT_SWEEP_INTERVAL` | `1m` | How often the turn-timeout sweeper runs. |
 | `BACKEND_GAME_CACHE_TTL` | `24h` | Idle window before a live game is evicted from cache. |
+| `BACKEND_LOBBY_ROBOT_WAIT` | `10s` | Auto-match wait before a robot is substituted for a missing human. |
+| `BACKEND_LOBBY_REAPER_INTERVAL` | `1s` | How often the substitution reaper scans for over-waited players. |
+| `BACKEND_ROBOT_DRIVE_INTERVAL` | `30s` | How often the robot driver scans for due robot turns. |
 | `BACKEND_SMTP_HOST` | — | Email relay host. **Empty selects the development log mailer** (the confirm-code is logged, not sent). |
 | `BACKEND_SMTP_PORT` | `587` | Email relay port. |
 | `BACKEND_SMTP_USERNAME` | — | SMTP user; empty relays without authentication. |