Fix failing test: revert quote-based discount to use sourceQuote.DiscountAmount

The quote discount must come from the agreed quote price, not the job's pricing
snapshot (which may have DiscountAmount=0 for legacy or unset reasons). The job
snapshot fix only applies to direct jobs where no source quote exists.

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

src/PowderCoating.Web/Controllers/InvoicesController.cs

@@ -469,11 +469,10 @@ public class InvoicesController : Controller
                         });
                     }
 
-                    // Use the quote's agreed tax rate. For discount, prefer the job's current pricing
-                    // snapshot — it is recalculated on every save and captures any post-conversion edits.
-                    // Fall back to the original quote discount only if no snapshot exists.
+                    // Use the quote's agreed tax rate and discount — these represent the customer-approved
+                    // price and must not be recomputed from the job's current state.
                     dto.TaxPercent     = sourceQuote.TaxPercent;
-                    dto.DiscountAmount = jobBreakdown?.DiscountAmount ?? sourceQuote.DiscountAmount;
+                    dto.DiscountAmount = sourceQuote.DiscountAmount;
                 }
                 else if (hadJobItems)
                 {