Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/model/api_key.go

@@ -0,0 +1,32 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package model // import "miniflux.app/v2/internal/model"
+
+import (
+	"time"
+
+	"miniflux.app/v2/internal/crypto"
+)
+
+// APIKey represents an application API key.
+type APIKey struct {
+	ID          int64
+	UserID      int64
+	Token       string
+	Description string
+	LastUsedAt  *time.Time
+	CreatedAt   time.Time
+}
+
+// NewAPIKey initializes a new APIKey.
+func NewAPIKey(userID int64, description string) *APIKey {
+	return &APIKey{
+		UserID:      userID,
+		Token:       crypto.GenerateRandomString(32),
+		Description: description,
+	}
+}
+
+// APIKeys represents a collection of API Key.
+type APIKeys []*APIKey