Fix grace period tests: set StripeSubscriptionId on test companies

Trial companies (no StripeSubscriptionId) get 0 grace days by design.
The GracePeriod and Expired status tests need a paid subscription to
exercise the 14-day grace window correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/PowderCoating.UnitTests/SubscriptionServiceTests.cs

@@ -144,6 +144,7 @@ public class SubscriptionServiceTests
             PrimaryContactEmail = "grace@example.com",
             SubscriptionStatus = SubscriptionStatus.Active,
             SubscriptionEndDate = DateTime.UtcNow.Date.AddDays(-5),
+            StripeSubscriptionId = "sub_grace_test",   // paid sub — qualifies for grace period
             IsActive = true
         });
         await context.SaveChangesAsync();
@@ -168,6 +169,7 @@ public class SubscriptionServiceTests
             PrimaryContactEmail = "expired@example.com",
             SubscriptionStatus = SubscriptionStatus.Active,
             SubscriptionEndDate = DateTime.UtcNow.Date.AddDays(-15),
+            StripeSubscriptionId = "sub_expired_test",  // paid sub — 15 days > 14-day grace window
             IsActive = true
         });
         await context.SaveChangesAsync();