Fix data purge FK violation on Appointments and apply pending migration

DataPurgeController was deleting Jobs without first clearing the nullable
Appointments.JobId FK, causing FK_Appointments_Jobs_JobId violations.
Fix nulls out the FK on any linked appointments before the DELETE runs.

Also applies migration AddAllowCustomFormulas (AllowCustomFormulas column
on SubscriptionPlanConfigs for custom formula pricing feature gating).

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

src/PowderCoating.Web/Controllers/DataPurgeController.cs

@@ -293,6 +293,16 @@ public class DataPurgeController : Controller
                 break;
 
             case "Jobs":
+                // Appointments.JobId is a nullable FK — null it out first so the DELETE
+                // doesn't violate FK_Appointments_Jobs_JobId.
+                var purgingJobIds = await _db.Jobs.IgnoreQueryFilters()
+                    .Where(e => e.IsDeleted && e.DeletedAt <= cutoff)
+                    .Select(e => e.Id)
+                    .ToListAsync();
+                if (purgingJobIds.Count > 0)
+                    await _db.Appointments.IgnoreQueryFilters()
+                        .Where(a => a.JobId.HasValue && purgingJobIds.Contains(a.JobId.Value))
+                        .ExecuteUpdateAsync(s => s.SetProperty(a => a.JobId, (int?)null));
                 count = await _db.Jobs.IgnoreQueryFilters()
                     .Where(e => e.IsDeleted && e.DeletedAt <= cutoff).ExecuteDeleteAsync();
                 break;