add mermaid code in setup

docs/DIRECTUS-SETUP.md

@@ -17,6 +17,89 @@ Komplette Anleitung zur Einrichtung von Directus als Backend für die dgray Klei
 
 ---
 
+## Datenmodell-Übersicht
+
+```mermaid
+erDiagram
+    USERS ||--o{ LISTINGS : creates
+    USERS ||--o{ FAVORITES : has
+    USERS ||--o{ CONVERSATIONS : participates
+    USERS ||--o{ MESSAGES : sends
+    USERS ||--o{ REPORTS : submits
+    
+    LISTINGS ||--o{ LISTINGS_FILES : has
+    LISTINGS ||--o{ FAVORITES : receives
+    LISTINGS ||--o{ CONVERSATIONS : has
+    LISTINGS }o--|| CATEGORIES : belongs_to
+    LISTINGS }o--o| LOCATIONS : located_at
+    
+    LISTINGS_FILES }o--|| DIRECTUS_FILES : references
+    
+    CATEGORIES ||--o{ CATEGORIES : has_children
+    CATEGORIES ||--o{ CATEGORIES_TRANSLATIONS : has
+    
+    CONVERSATIONS ||--o{ MESSAGES : contains
+    
+    LISTINGS {
+        uuid id PK
+        string status
+        string title
+        text description
+        decimal price
+        string currency
+        string condition
+        datetime expires_at
+        string monero_address
+    }
+    
+    CATEGORIES {
+        uuid id PK
+        string name
+        string slug
+        string icon
+        uuid parent FK
+    }
+    
+    CONVERSATIONS {
+        uuid id PK
+        uuid listing FK
+        uuid buyer FK
+        uuid seller FK
+        string status
+    }
+    
+    MESSAGES {
+        uuid id PK
+        uuid conversation FK
+        uuid sender FK
+        text content
+        datetime read_at
+    }
+    
+    FAVORITES {
+        uuid id PK
+        uuid user FK
+        uuid listing FK
+    }
+    
+    REPORTS {
+        uuid id PK
+        uuid reporter FK
+        uuid listing FK
+        string reason
+        string status
+    }
+    
+    LOCATIONS {
+        uuid id PK
+        string name
+        string postal_code
+        string country
+    }
+```
+
+---
+
 ## 1. Data Models (Collections)
 
 ### 1.1 listings (Anzeigen)