Add customergroup sort with bulk order and locked implicit names.

Reorder via customergroup.order (exact id list → sort 0…n-1); block renaming POS/Lieferanten while still allowing create/update of other groups by name.

src/schema.js

@@ -130,6 +130,7 @@ BEGIN
     CREATE TABLE customergroup (
         id INT IDENTITY(1,1) PRIMARY KEY,
         name NVARCHAR(255) NOT NULL,
+        sort INT NOT NULL CONSTRAINT DF_customergroup_sort DEFAULT 0,
         -- Nullable only for company-create chicken-egg (POS group first, then company, then SET).
         -- Always set before commit; never left null for live rows.
         company_id INT NULL,