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.Infrastructure/Migrations/20260524143607_MakeMaintenanceIntervalNullable.cs

@@ -0,0 +1,79 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace PowderCoating.Infrastructure.Migrations
+{
+    /// <inheritdoc />
+    public partial class MakeMaintenanceIntervalNullable : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AlterColumn<int>(
+                name: "RecommendedMaintenanceIntervalDays",
+                table: "Equipment",
+                type: "int",
+                nullable: true,
+                oldClrType: typeof(int),
+                oldType: "int");
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 1,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 24, 14, 36, 3, 317, DateTimeKind.Utc).AddTicks(8197));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 2,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 24, 14, 36, 3, 317, DateTimeKind.Utc).AddTicks(8203));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 3,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 24, 14, 36, 3, 317, DateTimeKind.Utc).AddTicks(8204));
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AlterColumn<int>(
+                name: "RecommendedMaintenanceIntervalDays",
+                table: "Equipment",
+                type: "int",
+                nullable: false,
+                defaultValue: 0,
+                oldClrType: typeof(int),
+                oldType: "int",
+                oldNullable: true);
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 1,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 23, 13, 51, 6, 293, DateTimeKind.Utc).AddTicks(4300));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 2,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 23, 13, 51, 6, 293, DateTimeKind.Utc).AddTicks(4313));
+
+            migrationBuilder.UpdateData(
+                table: "PricingTiers",
+                keyColumn: "Id",
+                keyValue: 3,
+                column: "CreatedAt",
+                value: new DateTime(2026, 5, 23, 13, 51, 6, 293, DateTimeKind.Utc).AddTicks(4315));
+        }
+    }
+}

src/PowderCoating.Infrastructure/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -3045,7 +3045,7 @@ namespace PowderCoating.Infrastructure.Migrations
                     b.Property<decimal>("PurchasePrice")
                         .HasColumnType("decimal(18,2)");
 
-                    b.Property<int>("RecommendedMaintenanceIntervalDays")
+                    b.Property<int?>("RecommendedMaintenanceIntervalDays")
                         .HasColumnType("int");
 
                     b.Property<string>("SerialNumber")
@@ -6711,7 +6711,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 1,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 23, 13, 51, 6, 293, DateTimeKind.Utc).AddTicks(4300),
+                            CreatedAt = new DateTime(2026, 5, 24, 14, 36, 3, 317, DateTimeKind.Utc).AddTicks(8197),
                             Description = "Standard pricing for regular customers",
                             DiscountPercent = 0m,
                             IsActive = true,
@@ -6722,7 +6722,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 2,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 23, 13, 51, 6, 293, DateTimeKind.Utc).AddTicks(4313),
+                            CreatedAt = new DateTime(2026, 5, 24, 14, 36, 3, 317, DateTimeKind.Utc).AddTicks(8203),
                             Description = "5% discount for preferred customers",
                             DiscountPercent = 5m,
                             IsActive = true,
@@ -6733,7 +6733,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 3,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 23, 13, 51, 6, 293, DateTimeKind.Utc).AddTicks(4315),
+                            CreatedAt = new DateTime(2026, 5, 24, 14, 36, 3, 317, DateTimeKind.Utc).AddTicks(8204),
                             Description = "10% discount for premium customers",
                             DiscountPercent = 10m,
                             IsActive = true,