refactor: introduce Wire dependency injection and interface-based architecture

- Add Google Wire for compile-time dependency injection
- Replace concrete service types with interfaces across handlers
- Remove global state (DB, cache) in favor of constructor injection
- Split monolithic files into focused modules:
  - config: separate files for each config domain
  - dto: converters split by domain (user, post, message, group)
  - cache: separate metrics.go and redis_cache.go
- Introduce unified apperrors package with string-based error codes
- Add transaction support with context-aware repository methods
- Upgrade golang.org/x/crypto from 0.17.0 to 0.26.0

internal/service/schedule_service.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"carrot_bbs/internal/dto"
+	apperrors "carrot_bbs/internal/errors"
 	"carrot_bbs/internal/model"
 	"carrot_bbs/internal/repository"
 
@@ -15,10 +16,10 @@ import (
 )
 
 var (
-	ErrInvalidSchedulePayload = &ServiceError{Code: 400, Message: "invalid schedule payload"}
-	ErrScheduleCourseNotFound = &ServiceError{Code: 404, Message: "schedule course not found"}
-	ErrScheduleForbidden      = &ServiceError{Code: 403, Message: "forbidden schedule operation"}
-	ErrScheduleColorDuplicated = &ServiceError{Code: 400, Message: "course color already used"}
+	ErrInvalidSchedulePayload  = apperrors.ErrInvalidSchedulePayload
+	ErrScheduleCourseNotFound  = apperrors.ErrScheduleCourseNotFound
+	ErrScheduleForbidden       = apperrors.ErrScheduleForbidden
+	ErrScheduleColorDuplicated = apperrors.ErrScheduleColorDuplicated
 )
 
 var hexColorRegex = regexp.MustCompile(`^#[0-9A-F]{6}$`)