R6(c): centralize shared integration-test fixtures in helpers.go

Move the cross-file integration fixtures — the service constructors
(newGameService/newSocialService/newRobotService/newMatchmaker), the game-assembly
helpers (newMirror/newGameWithSeats/newDraftGame), account provisioning
(provisionAccount/provisionGuest) and the stats reader — out of the domain test
files (newGameService alone was used by 10 files) into a single
backend/internal/inttest/helpers.go. Helpers used by a single file stay local.

Pure relocation: the helper bodies are unchanged, no test logic changes; the
imports the moves left unused are pruned. go vet -tags=integration is clean.

backend/internal/inttest/robot_test.go

@@ -8,31 +8,12 @@ import (
 	"time"
 
 	"github.com/google/uuid"
-	"go.opentelemetry.io/otel/metric/noop"
-	"go.uber.org/zap"
 
 	"scrabble/backend/internal/account"
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/game"
-	"scrabble/backend/internal/lobby"
-	"scrabble/backend/internal/robot"
 )
 
-// newRobotService builds a robot service over games (shared so its moves and the
-// test's human moves use the same live-game cache and per-game locks), a fresh
-// social service for nudges, and a no-op meter.
-func newRobotService(t *testing.T, games *game.Service) *robot.Service {
-	t.Helper()
-	return robot.NewService(games, account.NewStore(testDB), newSocialService(), noop.NewMeterProvider().Meter("robot-test"), zap.NewNop())
-}
-
-// newMatchmaker builds a matchmaker starting real games and substituting from
-// robots after wait.
-func newMatchmaker(t *testing.T, robots lobby.RobotProvider, wait time.Duration) *lobby.Matchmaker {
-	t.Helper()
-	return lobby.NewMatchmaker(newGameService(), robots, wait, zap.NewNop())
-}
-
 // setTurnStarted rewrites a game's turn clock so a robot turn can be made due (or
 // idle) at a chosen instant, independent of wall time.
 func setTurnStarted(t *testing.T, id uuid.UUID, at time.Time) {