feat: Mail template management UI and API CRUD

- Added full CRUD endpoints for mail templates (create, update, delete, preview)
- Introduced Joi validators for template create/update/preview
- Updated routes/controller to support ID and type lookups
- Built React Templates page with HTML editor, preview, and clipboard helpers
- Added navigation entry and route for /templates
- Enhanced documentation (README, QUICKSTART, KULLANIM, frontend/backend README)

backend/src/controllers/template.controller.js

@@ -1,6 +1,11 @@
 const { MailTemplate } = require('../models');
 const mailService = require('../services/mail.service');
 
+const buildTemplateResponse = (template) => ({
+  success: true,
+  data: template,
+});
+
 // Get all templates
 exports.getAllTemplates = async (req, res, next) => {
   try {
@@ -17,6 +22,25 @@ exports.getAllTemplates = async (req, res, next) => {
   }
 };
 
+// Get template by ID
+exports.getTemplateById = async (req, res, next) => {
+  try {
+    const { id } = req.params;
+    const template = await MailTemplate.findByPk(id);
+
+    if (!template) {
+      return res.status(404).json({
+        success: false,
+        error: 'Template not found',
+      });
+    }
+
+    res.json(buildTemplateResponse(template));
+  } catch (error) {
+    next(error);
+  }
+};
+
 // Get template by type
 exports.getTemplateByType = async (req, res, next) => {
   try {
@@ -33,9 +57,71 @@ exports.getTemplateByType = async (req, res, next) => {
       });
     }
 
+    res.json(buildTemplateResponse(template));
+  } catch (error) {
+    next(error);
+  }
+};
+
+// Create template
+exports.createTemplate = async (req, res, next) => {
+  try {
+    const template = await MailTemplate.create(req.body);
+    res.status(201).json(buildTemplateResponse(template));
+  } catch (error) {
+    if (error.name === 'SequelizeUniqueConstraintError') {
+      return res.status(400).json({
+        success: false,
+        error: 'Bu template_type değeri zaten kullanılıyor.',
+      });
+    }
+    next(error);
+  }
+};
+
+// Update template
+exports.updateTemplate = async (req, res, next) => {
+  try {
+    const { id } = req.params;
+
+    const template = await MailTemplate.findByPk(id);
+    if (!template) {
+      return res.status(404).json({
+        success: false,
+        error: 'Template not found',
+      });
+    }
+
+    await template.update(req.body);
+    res.json(buildTemplateResponse(template));
+  } catch (error) {
+    if (error.name === 'SequelizeUniqueConstraintError') {
+      return res.status(400).json({
+        success: false,
+        error: 'Bu template_type değeri zaten kullanılıyor.',
+      });
+    }
+    next(error);
+  }
+};
+
+// Delete template
+exports.deleteTemplate = async (req, res, next) => {
+  try {
+    const { id } = req.params;
+
+    const template = await MailTemplate.findByPk(id);
+    if (!template) {
+      return res.status(404).json({
+        success: false,
+        error: 'Template not found',
+      });
+    }
+
+    await template.destroy();
     res.json({
       success: true,
-      data: template,
+      message: 'Template deleted successfully',
     });
   } catch (error) {
     next(error);
@@ -68,5 +154,4 @@ exports.previewTemplate = async (req, res, next) => {
   }
 };
 
-module.exports = exports;
-
+module.exports = exports;