feat: add external SAM ID support to courses and update API documentation

services/lms-service/migrations/20260427000000_add_external_sam_id_to_courses.sql

@@ -0,0 +1,6 @@
+ALTER TABLE courses
+ADD COLUMN IF NOT EXISTS external_sam_id BIGINT;
+
+CREATE INDEX IF NOT EXISTS idx_courses_org_external_sam_id
+ON courses (organization_id, external_sam_id)
+WHERE external_sam_id IS NOT NULL;