Move internal packages to an internal folder

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

internal/model/category.go

@@ -0,0 +1,35 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package model // import "miniflux.app/v2/internal/model"
+
+import "fmt"
+
+// Category represents a feed category.
+type Category struct {
+	ID           int64  `json:"id"`
+	Title        string `json:"title"`
+	UserID       int64  `json:"user_id"`
+	HideGlobally bool   `json:"hide_globally"`
+	FeedCount    *int   `json:"feed_count,omitempty"`
+	TotalUnread  *int   `json:"total_unread,omitempty"`
+}
+
+func (c *Category) String() string {
+	return fmt.Sprintf("ID=%d, UserID=%d, Title=%s", c.ID, c.UserID, c.Title)
+}
+
+// CategoryRequest represents the request to create or update a category.
+type CategoryRequest struct {
+	Title        string `json:"title"`
+	HideGlobally string `json:"hide_globally"`
+}
+
+// Patch updates category fields.
+func (cr *CategoryRequest) Patch(category *Category) {
+	category.Title = cr.Title
+	category.HideGlobally = cr.HideGlobally != ""
+}
+
+// Categories represents a list of categories.
+type Categories []*Category