refactor: ensure all storage backends implement core.Storage

- Add import for `github.com/appleboy/gorush/core` in all storage backends
- Add type assertion to ensure `Storage` implements `core.Storage` in all storage backends

Signed-off-by: appleboy <appleboy.tw@gmail.com>

storage/boltdb/boltdb.go

@@ -5,9 +5,13 @@ import (
 	"os"
 	"sync"
 
+	"github.com/appleboy/gorush/core"
+
 	"github.com/asdine/storm/v3"
 )
 
+var _ core.Storage = (*Storage)(nil)
+
 // New func implements the storage interface for gorush (https://github.com/appleboy/gorush)
 func New(dbPath, bucket string) *Storage {
 	return &Storage{

storage/buntdb/buntdb.go

@@ -7,9 +7,13 @@ import (
 	"strconv"
 	"sync"
 
+	"github.com/appleboy/gorush/core"
+
 	"github.com/tidwall/buntdb"
 )
 
+var _ core.Storage = (*Storage)(nil)
+
 // New func implements the storage interface for gorush (https://github.com/appleboy/gorush)
 func New(dbPath string) *Storage {
 	return &Storage{

storage/leveldb/leveldb.go

@@ -6,9 +6,13 @@ import (
 	"strconv"
 	"sync"
 
+	"github.com/appleboy/gorush/core"
+
 	"github.com/syndtr/goleveldb/leveldb"
 )
 
+var _ core.Storage = (*Storage)(nil)
+
 func (s *Storage) setLevelDB(key string, count int64) {
 	value := fmt.Sprintf("%d", count)
 	_ = s.db.Put([]byte(key), []byte(value), nil)

storage/memory/memory.go

@@ -3,9 +3,13 @@ package memory
 import (
 	"sync"
 
+	"github.com/appleboy/gorush/core"
+
 	"go.uber.org/atomic"
 )
 
+var _ core.Storage = (*Storage)(nil)
+
 // New func implements the storage interface for gorush (https://github.com/appleboy/gorush)
 func New() *Storage {
 	return &Storage{}

storage/redis/redis.go

@@ -7,9 +7,13 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/appleboy/gorush/core"
+
 	"github.com/redis/go-redis/v9"
 )
 
+var _ core.Storage = (*Storage)(nil)
+
 // New func implements the storage interface for gorush (https://github.com/appleboy/gorush)
 func New(
 	addr string,