add new functionality and improve backend server structure

- Enhance server functionality and optimize backend code
- Add multi-database support (PostgreSQL, MySQL, SQLite)

server/core_backend-server/tools/cron/job.go

@@ -0,0 +1,41 @@
+package cron
+
+import "encoding/json"
+
+// Job defines a single registered cron job.
+type Job struct {
+	fn       func()
+	schedule *Schedule
+	id       string
+}
+
+// Id returns the cron job id.
+func (j *Job) Id() string {
+	return j.id
+}
+
+// Expression returns the plain cron job schedule expression.
+func (j *Job) Expression() string {
+	return j.schedule.rawExpr
+}
+
+// Run runs the cron job function.
+func (j *Job) Run() {
+	if j.fn != nil {
+		j.fn()
+	}
+}
+
+// MarshalJSON implements [json.Marshaler] and export the current
+// jobs data into valid JSON.
+func (j Job) MarshalJSON() ([]byte, error) {
+	plain := struct {
+		Id         string `json:"id"`
+		Expression string `json:"expression"`
+	}{
+		Id:         j.Id(),
+		Expression: j.Expression(),
+	}
+
+	return json.Marshal(plain)
+}