Add SMS START/re-subscribe handling to Twilio webhook

Customers who replied STOP by mistake can now reply START, YES, or
UNSTOP to automatically re-enable their SMS opt-in — no staff action
needed. Adds SmsInboundStart notification type, HandleStartAsync in
WebhooksController, and updates AI knowledge base and help docs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Core/Enums/NotificationEnums.cs

@@ -18,5 +18,6 @@ public enum NotificationType
     SubscriptionExpired = 11,
     SmsInboundStop = 12,
     SmsInboundHelp = 13,
-    AdminEmail = 14
+    AdminEmail = 14,
+    SmsInboundStart = 15
 }