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/social_test.go

@@ -45,29 +45,6 @@ func (c *capturePublisher) notified(user uuid.UUID, sub string) bool {
 	return false
 }
 
-// newSocialService builds a social service over the shared pool, reading game
-// state through a real game service.
-func newSocialService() *social.Service {
-	return social.NewService(social.NewStore(testDB), account.NewStore(testDB), newGameService())
-}
-
-// newGameWithSeats creates a started game seating n fresh accounts and returns the
-// game id and the seated account ids in seat order.
-func newGameWithSeats(t *testing.T, n int) (uuid.UUID, []uuid.UUID) {
-	t.Helper()
-	seats := make([]uuid.UUID, n)
-	for i := range seats {
-		seats[i] = provisionAccount(t)
-	}
-	g, err := newGameService().Create(context.Background(), game.CreateParams{
-		Variant: engine.VariantEnglish, Seats: seats, TurnTimeout: 24 * time.Hour, Seed: openingSeed(t),
-	})
-	if err != nil {
-		t.Fatalf("create game: %v", err)
-	}
-	return g.ID, seats
-}
-
 // TestFriendRequestToRobotStaysPending checks a friend request to a robot is accepted as
 // pending rather than blocked: robots no longer block friend requests, so the request
 // just sits unanswered and later expires — mirroring a human who ignores it.