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.Web/Views/Help/Settings.cshtml

@@ -416,7 +416,9 @@
                 <div>
                     Every outbound SMS automatically includes opt-out instructions ("Reply STOP to opt out"). If a
                     customer replies STOP, they are immediately opted out and will receive no further messages.
-                    You can re-enable them on their customer record if they later ask to be re-subscribed.
+                    If they change their mind, they can reply <strong>START</strong>, <strong>YES</strong>, or <strong>UNSTOP</strong>
+                    to re-subscribe automatically — no action needed from your staff. You can also manually re-enable
+                    SMS on their customer record.
                 </div>
             </div>
         </section>