feat(contents): add updateAdmin endpoint and repository method

- Introduced `PATCH /:id/admin` endpoint to update admin-specific content.
- Added `update` method to `ContentsRepository` for data updates with timestamp.

backend/src/contents/repositories/contents.repository.ts

@@ -404,6 +404,15 @@ export class ContentsRepository {
 		return deleted;
 	}
 
+	async update(id: string, data: Partial<typeof contents.$inferInsert>) {
+		const [updated] = await this.databaseService.db
+			.update(contents)
+			.set({ ...data, updatedAt: new Date() })
+			.where(eq(contents.id, id))
+			.returning();
+		return updated;
+	}
+
 	async findBySlug(slug: string) {
 		const [result] = await this.databaseService.db
 			.select()