fix: migration 0029 enum DO-Block statt CREATE TYPE IF NOT EXISTS

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

backend/migrations/versions/0029_special_assignments.py

@@ -23,8 +23,15 @@ depends_on = None
 def upgrade() -> None:
     op.execute("CREATE EXTENSION IF NOT EXISTS btree_gist")
 
-    # Enum erzeugen (IF NOT EXISTS: falls create_all in lifespan den Typ schon angelegt hat)
-    op.execute("CREATE TYPE IF NOT EXISTS assignment_mode AS ENUM ('fza', 'payroll', 'both')")
+    # Enum erzeugen (nur falls nicht vorhanden – CREATE TYPE IF NOT EXISTS nicht überall unterstützt)
+    op.execute("""
+        DO $$
+        BEGIN
+            IF NOT EXISTS (SELECT 1 FROM pg_type WHERE typname = 'assignment_mode') THEN
+                CREATE TYPE assignment_mode AS ENUM ('fza', 'payroll', 'both');
+            END IF;
+        END $$
+    """)
 
     # Tabelle nur anlegen falls noch nicht vorhanden (idempotent)
     op.execute("""