fix: add migration to update existing tier change template (refs #59)

The original migration already inserted the template with Handlebars conditionals.
This migration updates the existing record to use simple variable substitution.

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

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

@@ -0,0 +1,26 @@
+/**
+ * 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';