Add facility overhead (rent + utilities) to operating costs and pricing engine

Adds MonthlyRent, MonthlyUtilities, and MonthlyBillableHours to CompanyOperatingCosts so fixed shop occupancy costs are recovered on every quote. The pricing engine converts these into a per-hour rate and applies it as a transparent "Facility Overhead" line between oven batch cost and shop supplies. UI added in Company Settings Operating Costs tab and Setup Wizard Step 3; migration AddFacilityOverheadFields applied. Help docs and AI knowledge base updated to cover the new fields and the revised quote pricing calculation order.

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

src/PowderCoating.Core/Entities/CompanyOperatingCosts.cs

@@ -127,5 +127,24 @@ namespace PowderCoating.Core.Entities
         /// </summary>
         [Range(0, 5000)]
         public decimal? CoatingRateSqFtPerHourOverride { get; set; }
+
+        // ── Facility Overhead ─────────────────────────────────────────────────────
+        // Monthly fixed costs divided by billable hours to derive a per-hour overhead
+        // rate, which is then applied to each quote based on estimated labor time.
+
+        /// <summary>Monthly shop lease / rent payment.</summary>
+        [Range(0, 1000000)]
+        public decimal MonthlyRent { get; set; } = 0m;
+
+        /// <summary>Monthly utilities combined (electricity, gas, water, internet).</summary>
+        [Range(0, 1000000)]
+        public decimal MonthlyUtilities { get; set; } = 0m;
+
+        /// <summary>
+        /// Estimated billable shop hours per month used to amortise facility overhead.
+        /// Defaults to 160 (4 weeks × 40 hrs). Must be at least 1 to avoid division by zero.
+        /// </summary>
+        [Range(1, 10000)]
+        public int MonthlyBillableHours { get; set; } = 160;
     }
 }