fix: production JSON parsing errors and trust proxy issue

- Set Express to trust proxy headers for proper IP detection with Traefik
- Add safe JSON parsing for email template variables and activity log metadata
- Create migration to fix invalid JSON data in database
- Add error handling to prevent JSON.parse crashes

This fixes the 500 errors caused by invalid JSON data and the trust proxy
warning from express-rate-limit when running behind Traefik.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/src/routes/adminEmail.js

@@ -164,7 +164,16 @@ router.get('/templates', adminAuth, async (req, res) => {
       const result = {
         id: template.id,
         template_key: template.template_key,
-        variables: template.variables ? JSON.parse(template.variables) : [],
+        variables: (() => {
+        try {
+          if (!template.variables) return [];
+          if (typeof template.variables === 'object') return template.variables;
+          return JSON.parse(template.variables);
+        } catch (e) {
+          console.warn('Failed to parse variables for template:', template.template_key, e.message);
+          return [];
+        }
+      })(),
         updated_at: template.updated_at
       };
       
@@ -212,7 +221,16 @@ router.get('/templates/:key', adminAuth, async (req, res) => {
     const response = {
       id: template.id,
       template_key: template.template_key,
-      variables: template.variables ? JSON.parse(template.variables) : [],
+      variables: (() => {
+        try {
+          if (!template.variables) return [];
+          if (typeof template.variables === 'object') return template.variables;
+          return JSON.parse(template.variables);
+        } catch (e) {
+          console.warn('Failed to parse variables for template:', template.template_key, e.message);
+          return [];
+        }
+      })(),
       updated_at: template.updated_at
     };