fix: use upsert for tier change template migration (refs #59)

Changed INSERT to INSERT...ON CONFLICT DO UPDATE so the migration works for:
- Fresh deployments (inserts new template)
- Existing databases (updates template to fix variable substitution)

Removed unnecessary migration 008.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/features/notifications/migrations/007_subscription_tier_change_template.sql

@@ -17,7 +17,7 @@ CHECK (template_key IN (
   'subscription_tier_change'
 ));
 
--- Insert subscription tier change email template
+-- Insert or update subscription tier change email template
 INSERT INTO email_templates (template_key, name, description, subject, body, variables, html_body) VALUES
   (
     'subscription_tier_change',
@@ -86,4 +86,12 @@ MotoVaultPro Team',
   </table>
 </body>
 </html>'
-  );
+  )
+ON CONFLICT (template_key) DO UPDATE SET
+  name = EXCLUDED.name,
+  description = EXCLUDED.description,
+  subject = EXCLUDED.subject,
+  body = EXCLUDED.body,
+  variables = EXCLUDED.variables,
+  html_body = EXCLUDED.html_body,
+  updated_at = NOW();

backend/src/features/notifications/migrations/008_fix_tier_change_template.sql

@@ -1,26 +0,0 @@
-/**
- * Migration: Fix subscription tier change email template
- * @ai-summary Fixes template to use simple variable substitution instead of conditionals
- * @ai-context TemplateService only supports {{variable}}, not Handlebars conditionals
- */
-
--- Update the existing template to use simple variable substitution
-UPDATE email_templates
-SET
-  body = 'Hi {{userName}},
-
-Your MotoVaultPro subscription has been {{changeType}}.
-
-Previous Tier: {{previousTier}}
-New Tier: {{newTier}}
-Reason: {{reason}}
-
-{{additionalInfo}}
-
-If you have any questions, please contact support.
-
-Best regards,
-MotoVaultPro Team',
-  variables = '["userName", "changeType", "previousTier", "newTier", "reason", "additionalInfo"]',
-  updated_at = NOW()
-WHERE template_key = 'subscription_tier_change';