Refactor: Name-Feld entfernen, Email als einziger Identifier

- Name-Spalte aus Nutzertabelle entfernt
- Anzeigename-Feld aus Nutzer-Anlegen-Dialog entfernt
- Username wird automatisch aus Email-Prefix generiert
- UserCreate Model: username jetzt optional

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/routers/users.py

@@ -66,11 +66,12 @@ async def create_user(
     if await cursor.fetchone():
         raise HTTPException(status_code=400, detail="E-Mail bereits vergeben")
 
+    username = data.username if data.username else email.split("@")[0]
     now = datetime.now(timezone.utc).isoformat()
     cursor = await db.execute(
         """INSERT INTO users (email, username, password_hash, organization_id, role, is_active, created_at)
            VALUES (?, ?, '', ?, ?, 1, ?)""",
-        (email, data.username, org_id, data.role, now),
+        (email, username, org_id, data.role, now),
     )
     user_id = cursor.lastrowid
 
@@ -91,7 +92,7 @@ async def create_user(
         from email_utils.sender import send_email
         from email_utils.templates import invite_email
         link = f"{MAGIC_LINK_BASE_URL}/auth/verify?token={token}"
-        subject, html = invite_email(data.username, org["name"], code, link)
+        subject, html = invite_email(username, org["name"], code, link)
         await send_email(email, subject, html)
     except Exception:
         pass  # E-Mail-Fehler nicht fatal