PR 3: Observability back-links and terminology cleanup

- Added an Observability back-link at the top of all 7 Observability-area pages (AuditLog, SystemLogs, SystemInfo, AiUsageReport, UsageQuota, BannedIps, Diagnostics/Index) consistent with the Maintenance back-link pattern from PR 2
- Renamed company-level nav label and view title from "Notification Log" to "Email & SMS Log" to distinguish it clearly from the platform-level "Platform Notifications", "Audit Log", and "System Logs" surfaces

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

src/PowderCoating.Web/Views/AuditLog/Index.cshtml

@@ -37,6 +37,11 @@
 }
 
 <div class="container-fluid py-3">
+    <div class="mb-2">
+        <a asp-controller="PlatformAdmin" asp-action="Observability" class="text-muted small text-decoration-none">
+            <i class="bi bi-arrow-left me-1"></i>Observability
+        </a>
+    </div>
     <div class="d-flex align-items-center justify-content-between mb-3">
         <div>
             <h4 class="mb-0"><i class="bi bi-shield-check me-2 text-primary"></i>Audit Log</h4>