Add UML class & sequence diagrams

docs/uml/classes_SimpleWebapp.puml

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+@startuml class_diagram
+
+class User {
+  - id: int
+  - username: str
+  - password: str
+  + is_authenticated(): bool
+}
+
+class Item {
+  - id: int
+  - title: str
+  - description: str
+  + save(): void
+}
+
+class AuthController {
+  + login(username: str, password: str): bool
+  + logout(): void
+}
+
+class ItemController {
+  + create_item(data): Item
+  + get_item_by_id(item_id: int): Item
+}
+
+User "1" -- "0..*" Item : owns
+AuthController --> User : authenticates
+ItemController --> Item : manages
+
+@enduml

docs/uml/sequence_create_item.puml

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+@startuml sequence_item
+actor User
+participant "ItemController" as IC
+database "Database" as DB
+entity "Item Model" as IM
+
+User -> IC: POST /item/new (title, description)
+IC -> IM: Create Item(data)
+IM -> DB: INSERT item
+DB --> IM: Success
+IM --> IC: Item Created
+IC -> User: Redirect to item page
+@enduml

docs/uml/sequence_login.puml

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+@startuml sequence_login
+actor User
+participant "AuthController" as AC
+database "Database" as DB
+entity "User Model" as UM
+
+User -> AC: POST /login (username, password)
+AC -> DB: Query user by username
+DB --> AC: User data
+AC -> UM: Validate password
+UM --> AC: True/False
+alt Valid
+    AC -> User: Redirect to Dashboard
+else Invalid
+    AC -> User: Show error message
+end
+@enduml