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/mails/base.go

@@ -0,0 +1,33 @@
+// Package mails implements various helper methods for sending common
+// emails like forgotten password, verification, etc.
+package mails
+
+import (
+	"bytes"
+	"text/template"
+)
+
+// resolveTemplateContent resolves inline html template strings.
+func resolveTemplateContent(data any, content ...string) (string, error) {
+	if len(content) == 0 {
+		return "", nil
+	}
+
+	t := template.New("inline_template")
+
+	var parseErr error
+	for _, v := range content {
+		t, parseErr = t.Parse(v)
+		if parseErr != nil {
+			return "", parseErr
+		}
+	}
+
+	var wr bytes.Buffer
+
+	if executeErr := t.Execute(&wr, data); executeErr != nil {
+		return "", executeErr
+	}
+
+	return wr.String(), nil
+}