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/Helpers/HelpKnowledgeBase.cs

@@ -1097,8 +1097,8 @@ public static class HelpKnowledgeBase
         **Send SMS button:**
         On any completed job's Details page, Company Admins and Managers see a **Send SMS** button that opens the same compose modal, allowing you to send a follow-up message at any time.
 
-        **Customer opt-out:**
-        If a customer replies STOP to any message, they are automatically opted out and will not receive further SMS messages. They can reply START to re-subscribe.
+        **Customer opt-out and opt back in:**
+        If a customer replies STOP (or STOPALL, CANCEL, END, QUIT, UNSUBSCRIBE) to any message, they are automatically opted out and will not receive further SMS messages. If they change their mind, they can reply START, YES, or UNSTOP to re-subscribe automatically — no action needed from your staff. You can also manually toggle SMS back on from their customer record.
 
         ### Platform Announcements
         Occasional platform-wide announcements from the Powder Coating Logix team are delivered directly to your notification bell — not as page banners. These may cover new features, scheduled maintenance, or policy updates. They appear as **Announcement** type items in the bell dropdown.