Add mermaid diagrams of before (DbBase inheritance) and after (DTO + mappers) situation

after-dto.mmd

@@ -0,0 +1,45 @@
+%% After: Client and Order become plain domain objects (no DbBase inheritance).
+%% Dedicated *Dto classes carry data to/from the WebApp, and a mapper converts
+%% each way, including the relations. EF dependency isolated to persistence layer.
+classDiagram
+    class DbContext {
+        +SaveChangesAsync()
+    }
+    class DbBase {
+        +Guid id
+    }
+    class WebApp {
+        <<Web App>>
+    }
+    class Client {
+        +Guid id
+        +string name
+    }
+    class Order {
+        +Guid id
+    }
+    class ClientDto {
+        +Guid id
+        +string name
+    }
+    class OrderDto {
+        +Guid id
+        +ClientDto client
+    }
+    class Mappers {
+        +ClientDto ToClientDto(Client c)
+        +Client FromClientDto(ClientDto dto)
+        +OrderDto ToOrderDto(Order o)
+        +Order FromOrderDto(OrderDto dto)
+    }
+
+    DbContext ..> DbBase : still used by persistence layer
+    WebApp --> ClientDto
+    WebApp --> OrderDto
+    WebApp --> Mappers
+    Mappers <--> Client : maps each way
+    Mappers <--> ClientDto : maps each way
+    Mappers <--> Order : maps each way
+    Mappers <--> OrderDto : maps each way
+    Client "1" --> "*" Order : has many (domain relation)
+    ClientDto "1" --> "*" OrderDto : has many (DTO relation)

before-after-dto.mmd

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+%% Before: WebApp's domain classes inherit from DbBase (EF),
+%% which depends on DbContext. Relations live inside the DB-backed classes.
+classDiagram
+    class DbContext {
+        +SaveChangesAsync()
+    }
+    class DbBase {
+        +Guid id
+    }
+    class WebApp {
+        <<Web App>>
+    }
+    class Client {
+        +Guid id
+        +string name
+    }
+    class Order {
+        +Guid id
+    }
+
+    DbBase --> DbContext : uses (EF)
+    WebApp ..> DbBase : depends on
+    Client --|> DbBase : inherits
+    Order --|> DbBase : inherits
+    WebApp --> Client
+    WebApp --> Order
+    Client "1" --> "*" Order : has many
+    Order "*" --> "1" Client : open / close