docs: adds country

docs/database_schema.dbml

@@ -131,6 +131,7 @@ Table members {
   street text [null, note: 'Street name']
   house_number text [null, note: 'House number']
   postal_code text [null, note: '5-digit German postal code']
+  country text [null, note: 'Country of residence']
   search_vector tsvector [null, note: 'Full-text search index (auto-generated)']
   membership_fee_type_id uuid [null, note: 'FK to membership_fee_types - assigned fee type']
   membership_fee_start_date date [null, note: 'Date from which membership fees should be calculated']
@@ -189,6 +190,7 @@ Table members {
     - join_date: cannot be in future
     - exit_date: must be after join_date (if both present)
     - postal_code: optional (no format validation)
+    - country: optional
   '''
 }