Initial commit MKD fixes

backend/migrations/create_employee_districts.sql

@@ -0,0 +1,18 @@
+-- Связь сотрудник — участки (многие ко многим): один сотрудник может быть назначен на несколько участков
+CREATE TABLE IF NOT EXISTS employee_districts (
+  employee_id VARCHAR(50) NOT NULL REFERENCES employees(id) ON DELETE CASCADE,
+  district_id VARCHAR(50) NOT NULL REFERENCES districts(id) ON DELETE CASCADE,
+  created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  PRIMARY KEY (employee_id, district_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_employee_districts_employee ON employee_districts(employee_id);
+CREATE INDEX IF NOT EXISTS idx_employee_districts_district ON employee_districts(district_id);
+COMMENT ON TABLE employee_districts IS 'Назначения сотрудников на участки; один сотрудник может быть на нескольких участках';
+
+-- Перенос существующих назначений из employees.assigned_district_id
+INSERT INTO employee_districts (employee_id, district_id)
+SELECT id, assigned_district_id
+FROM employees
+WHERE assigned_district_id IS NOT NULL AND assigned_district_id <> ''
+ON CONFLICT (employee_id, district_id) DO NOTHING;