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/account_test.go

@@ -314,19 +314,6 @@ func TestNotificationsInAppOnlyRoundTrip(t *testing.T) {
 	}
 }
 
-// provisionGuest creates a fresh ephemeral guest account and returns its id.
-func provisionGuest(t *testing.T) uuid.UUID {
-	t.Helper()
-	acc, err := account.NewStore(testDB).ProvisionGuest(context.Background())
-	if err != nil {
-		t.Fatalf("provision guest: %v", err)
-	}
-	if !acc.IsGuest {
-		t.Fatalf("provisioned account %s is not flagged guest", acc.ID)
-	}
-	return acc.ID
-}
-
 // TestGuestAutoMatchLeavesNoStats drives a guest through a full auto-match game
 // against a robot to a natural end and checks the guest holds a seat (the
 // game_players foreign key is satisfied) yet accrues no statistics, while the