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/Controllers/WebhooksController.cs

@@ -31,6 +31,12 @@ public class WebhooksController : ControllerBase
         "STOP", "STOPALL", "UNSUBSCRIBE", "CANCEL", "END", "QUIT"
     };
 
+    // CTIA-standard opt-in keywords
+    private static readonly HashSet<string> StartKeywords = new(StringComparer.OrdinalIgnoreCase)
+    {
+        "START", "YES", "UNSTOP"
+    };
+
     // CTIA-standard help keywords
     private static readonly HashSet<string> HelpKeywords = new(StringComparer.OrdinalIgnoreCase)
     {
@@ -77,6 +83,9 @@ public class WebhooksController : ControllerBase
         if (StopKeywords.Contains(body))
             return await HandleStopAsync(payload.From);
 
+        if (StartKeywords.Contains(body))
+            return await HandleStartAsync(payload.From);
+
         if (HelpKeywords.Contains(body))
             return await HandleHelpAsync(payload.From);
 
@@ -123,6 +132,44 @@ public class WebhooksController : ControllerBase
             $"Reply START to re-subscribe.");
     }
 
+    // ── START ─────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Processes a START keyword: re-enables SMS for the customer, clears SmsOptedOutAt,
+    /// logs to NotificationLog, and returns a TwiML confirmation message.
+    /// </summary>
+    private async Task<IActionResult> HandleStartAsync(string from)
+    {
+        var (customer, digits10) = await FindCustomerByPhoneAsync(from);
+
+        if (customer == null)
+        {
+            _logger.LogWarning("Twilio START from {From} — no matching customer found", from);
+            return TwimlMessage("You have been re-subscribed and will receive messages again.");
+        }
+
+        var companyName = await GetCompanyNameAsync(customer.CompanyId);
+
+        if (!customer.NotifyBySms)
+        {
+            customer.NotifyBySms   = true;
+            customer.SmsOptedOutAt = null;
+            customer.UpdatedAt     = DateTime.UtcNow;
+            await _context.SaveChangesAsync();
+            _logger.LogInformation("Customer {CustomerId} re-subscribed to SMS via START reply", customer.Id);
+        }
+
+        await WriteInboundLogAsync(
+            NotificationType.SmsInboundStart,
+            customer,
+            from,
+            "START");
+
+        return TwimlMessage(
+            $"{companyName}: You have been re-subscribed and will receive messages again. " +
+            $"Reply STOP to unsubscribe at any time.");
+    }
+
     // ── HELP ──────────────────────────────────────────────────────────────────
 
     /// <summary>