fix: Change completedAt from nullable to optional to fix TypeScript error

- Update CourseProgressSchema to use .optional() instead of .nullable()
- This changes the type from string | null to string | undefined
- Fixes CI type-check error on line 53 of certificate-service.ts

Author: jotel-dev

src/schemas/progress.schema.ts

@@ -17,7 +17,7 @@ export const CourseProgressSchema = z.object({
   progress: z.number().min(0).max(100),
   completedLessons: z.array(z.string()),
   lastAccessedAt: z.string(),
-  completedAt: z.string().nullable(),
+  completedAt: z.string().optional(),
 });
 
 export type CourseProgress = z.infer<typeof CourseProgressSchema>;