refactor: extract lesson completion logic into shared markLessonComplete method and add course progress tracking to video and quiz responses

Backend/src/types/CoursesStudent.types.ts

@@ -242,6 +242,8 @@ export interface SaveVideoProgressResponse {
         video_progress_percentage: number | null;
         is_completed: boolean;
         last_watched_at: Date;
+        course_progress_percentage?: number;
+        is_course_completed?: boolean;
     };
 }
 
@@ -362,5 +364,7 @@ export interface SubmitQuizResponse {
             is_correct: boolean;
             score: number;
         }[];
+        course_progress_percentage?: number;
+        is_course_completed?: boolean;
     };
 }