Update Development Guide

Development-Guide.md

@@ -44,7 +44,7 @@ go test -v ./...
 - `handler/` — HTTP concerns only. Decode the request body, call a service method, encode the response. No business logic here.
 - `service/` — All business logic. Favourability calculation, variance rollup, budget rules. No SQL here.
 - `database/` — All SQL. Repository structs implement interfaces used by the service layer.
-- `model/` — Plain Go structs shared across layers. No methods with business logic.
+- `model/` — Plain Go structs shared across layers. No methods with business logic, only struct validation.
  
 **Error handling:**