Fix equipment creation blocked by maintenance interval validation

RecommendedMaintenanceIntervalDays was a non-nullable int with [Range(1,3650)],
so submitting the form without filling it in bound to 0 and failed validation.
Made nullable on the entity, both DTOs, and the one controller callsite that
calls .AddDays() (now uses .Value). Migration applied.

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

src/PowderCoating.Core/Entities/Equipment.cs

@@ -19,7 +19,7 @@ public class Equipment : BaseEntity
     public string? Location { get; set; }
     
     // Maintenance Information
-    public int RecommendedMaintenanceIntervalDays { get; set; }
+    public int? RecommendedMaintenanceIntervalDays { get; set; }
     public DateTime? LastMaintenanceDate { get; set; }
     public DateTime? NextScheduledMaintenance { get; set; }