added docs

docs/architecture-class-diagram.puml

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+@startuml "Backend Klassendiagramm"
+
+skinparam packageStyle rectangle
+skinparam classAttributeIconSize 0
+skinparam classFontSize 11
+skinparam defaultFontSize 10
+
+top to bottom direction
+
+title Backend Architektur - Klassendiagramm
+
+package "Controller Layer" #ADD8E6 {
+    class AuthController {
+        ' +login()
+        ' +register()
+        ' +refresh()
+        ' +logout()
+    }
+
+    class ChatController {
+        ' +sendMessage()
+        ' +confirmEvent()
+        ' +rejectEvent()
+        ' +getConversations()
+    }
+
+    class EventController {
+        ' +create()
+        ' +getAll()
+        ' +getByDateRange()
+        ' +update()
+        ' +delete()
+    }
+
+    class AuthMiddleware {
+        ' +authenticate()
+    }
+}
+
+package "Service Layer" #90EE90 {
+    package "Interfaces" {
+        interface AIProvider {
+            ' +processMessage()
+        }
+
+        interface UserRepository {
+            ' +findById()
+            ' +findByEmail()
+            ' +create()
+        }
+
+        interface EventRepository {
+            ' +findById()
+            ' +findByUserId()
+            ' +findByDateRange()
+            ' +create()
+            ' +update()
+            ' +delete()
+        }
+
+        interface ChatRepository {
+            ' +getHistory()
+            ' +create()
+        }
+    }
+
+    class AuthService {
+        ' -userRepo: UserRepository
+        ' +login()
+        ' +register()
+    }
+
+    class ChatService {
+        ' -chatRepo: ChatRepository
+        ' -eventRepo: EventRepository
+        ' -aiProvider: AIProvider
+        ' +processMessage()
+        ' +confirmEvent()
+    }
+
+    class EventService {
+        ' -eventRepo: EventRepository
+        ' +create()
+        ' +getByDateRange()
+        ' +update()
+        ' +delete()
+    }
+}
+
+package "AI Layer" #FFA07A {
+    class ClaudeAdapter implements AIProvider {
+        ' -apiKey: string
+        ' +processMessage()
+    }
+}
+
+package "Data Access Layer" #FFD700 {
+    class MongoUserRepository implements UserRepository {
+        ' -model: UserModel
+    }
+
+    class MongoEventRepository implements EventRepository {
+        ' -model: EventModel
+    }
+
+    class MongoChatRepository implements ChatRepository {
+        ' -model: ChatModel
+    }
+}
+
+package "Models" #D3D3D3 {
+    class User <<Entity>> {
+        ' +id: string
+        ' +email: string
+        ' +displayName: string
+    }
+
+    class CalendarEvent <<Entity>> {
+        ' +id: string
+        ' +userId: string
+        ' +title: string
+        ' +startTime: Date
+        ' +endTime: Date
+        ' +note?: string
+    }
+
+    class ChatMessage <<Entity>> {
+        ' +id: string
+        ' +sender: string
+        ' +content: string
+        ' +proposedEvent?: ProposedEvent
+    }
+}
+
+package "Utils" #DDA0DD {
+    class JWT {
+        ' +signToken()
+        ' +verifyToken()
+    }
+
+    class Password {
+        ' +hash()
+        ' +compare()
+    }
+}
+
+' Controller -> Service
+AuthController --> AuthService
+ChatController --> ChatService
+EventController --> EventService
+AuthMiddleware --> JWT
+
+' Service -> Interfaces (intern)
+AuthService --> UserRepository
+ChatService --> ChatRepository
+ChatService --> EventRepository
+ChatService --> AIProvider
+EventService --> EventRepository
+
+' Auth uses Utils
+AuthService --> JWT
+AuthService --> Password
+
+@enduml