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/draft_test.go

@@ -5,35 +5,10 @@ package inttest
 import (
 	"context"
 	"testing"
-	"time"
 
-	"github.com/google/uuid"
-
-	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/game"
 )
 
-// newDraftGame creates a started two-player English game on an opening seed and returns the
-// service, game id, seats, and the opening play (from a mirror) used to drive a real commit.
-func newDraftGame(t *testing.T) (*game.Service, uuid.UUID, []uuid.UUID, engine.MoveRecord) {
-	t.Helper()
-	ctx := context.Background()
-	svc := newGameService()
-	seats := []uuid.UUID{provisionAccount(t), provisionAccount(t)}
-	seed := openingSeed(t)
-	g, err := svc.Create(ctx, game.CreateParams{
-		Variant: engine.VariantEnglish, Seats: seats, TurnTimeout: 24 * time.Hour, Seed: seed,
-	})
-	if err != nil {
-		t.Fatalf("create: %v", err)
-	}
-	hint, ok := newMirror(t, seed, 2).HintView()
-	if !ok || len(hint.Tiles) == 0 {
-		t.Fatal("no opening move")
-	}
-	return svc, g.ID, seats, hint
-}
-
 // TestDraftPersistAndConflictReset covers draft persistence: a round-trip of the
 // rack order + board tiles, the actor's own draft cleared on their move, and an opponent's
 // board draft reset when a committed play overlaps one of its cells (the rack order kept).