Move internal packages to an internal folder

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

internal/model/app_session.go

@@ -0,0 +1,58 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package model // import "miniflux.app/v2/internal/model"
+
+import (
+	"database/sql/driver"
+	"encoding/json"
+	"errors"
+	"fmt"
+)
+
+// SessionData represents the data attached to the session.
+type SessionData struct {
+	CSRF               string `json:"csrf"`
+	OAuth2State        string `json:"oauth2_state"`
+	FlashMessage       string `json:"flash_message"`
+	FlashErrorMessage  string `json:"flash_error_message"`
+	Language           string `json:"language"`
+	Theme              string `json:"theme"`
+	PocketRequestToken string `json:"pocket_request_token"`
+}
+
+func (s SessionData) String() string {
+	return fmt.Sprintf(`CSRF=%q, OAuth2State=%q, FlashMsg=%q, FlashErrMsg=%q, Lang=%q, Theme=%q, PocketTkn=%q`,
+		s.CSRF, s.OAuth2State, s.FlashMessage, s.FlashErrorMessage, s.Language, s.Theme, s.PocketRequestToken)
+}
+
+// Value converts the session data to JSON.
+func (s SessionData) Value() (driver.Value, error) {
+	j, err := json.Marshal(s)
+	return j, err
+}
+
+// Scan converts raw JSON data.
+func (s *SessionData) Scan(src interface{}) error {
+	source, ok := src.([]byte)
+	if !ok {
+		return errors.New("session: unable to assert type of src")
+	}
+
+	err := json.Unmarshal(source, s)
+	if err != nil {
+		return fmt.Errorf("session: %v", err)
+	}
+
+	return err
+}
+
+// Session represents a session in the system.
+type Session struct {
+	ID   string
+	Data *SessionData
+}
+
+func (s *Session) String() string {
+	return fmt.Sprintf(`ID="%s", Data={%v}`, s.ID, s.Data)
+}