refactor: replace standard log with zap logger and optimize code patterns

- Migrate all log.Printf/log.Println calls to structured zap logging
- Use cmp.Or for cleaner default value handling
- Replace manual loops with slices.ContainsFunc/IndexFunc
- Add batch query methods (IsLikedBatch, IsFavoritedBatch) to solve N+1 problem
- Update JSON tags from omitempty to omitzero for numeric fields
- Use errgroup-style wg.Go for worker goroutines
- Remove duplicate casbin/v2 dependency (keeping v3)
- Add plans/ to gitignore

internal/dto/schedule_dto.go

@@ -5,9 +5,9 @@ type ScheduleCourseResponse struct {
 	Name         string `json:"name"`
 	Teacher      string `json:"teacher,omitempty"`
 	Location     string `json:"location,omitempty"`
-	DayOfWeek    int    `json:"day_of_week"`
-	StartSection int    `json:"start_section"`
-	EndSection   int    `json:"end_section"`
+	DayOfWeek    int    `json:"day_of_week,omitzero"`
+	StartSection int    `json:"start_section,omitzero"`
+	EndSection   int    `json:"end_section,omitzero"`
 	Weeks        []int  `json:"weeks"`
 	Color        string `json:"color,omitempty"`
 }