Add audit system, invite links, and usage analytics (#4749)

# Description of Changes

New Features
Audit System: Complete audit logging with dashboard, event tracking, and
export capabilities

Invite Links: Secure invite system with email notifications and
expiration

Usage Analytics: Endpoint usage statistics and visualization

License Management: User counting with grandfathering and license
enforcement
## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: James Brunton <[email protected]>

frontend/src/core/hooks/useAdminSettings.ts

@@ -71,15 +71,15 @@ export function useAdminSettings<T = any>(
       // Store raw settings (includes _pending if present)
       setRawSettings(rawData);
 
-      // Extract active settings (without _pending) for delta comparison
-      const { _pending, ...activeOnly } = rawData;
-      setOriginalSettings(activeOnly as T);
-      console.log(`[useAdminSettings:${sectionName}] Original active settings:`, JSON.stringify(activeOnly, null, 2));
-
       // Merge pending changes into settings for display
       const mergedSettings = mergePendingSettings(rawData);
       console.log(`[useAdminSettings:${sectionName}] Merged settings:`, JSON.stringify(mergedSettings, null, 2));
 
+      // Store merged settings as original for delta comparison
+      // This ensures we compare against what the user SAW (with pending), not raw active values
+      setOriginalSettings(mergedSettings as T);
+      console.log(`[useAdminSettings:${sectionName}] Original settings (for comparison):`, JSON.stringify(mergedSettings, null, 2));
+
       setSettings(mergedSettings as T);
     } catch (error) {
       console.error(`[useAdminSettings:${sectionName}] Failed to fetch:`, error);
@@ -106,15 +106,46 @@ export function useAdminSettings<T = any>(
         // Use custom save logic for complex sections
         const { sectionData, deltaSettings } = saveTransformer(settings);
 
-        // Save section data (with delta applied)
-        const sectionDelta = computeDelta(originalSettings, sectionData);
+        // Get original sectionData using same transformer for fair comparison
+        const { sectionData: originalSectionData } = saveTransformer(originalSettings);
+
+        // Save section data (with delta applied) - compare transformed vs transformed
+        const sectionDelta = computeDelta(originalSectionData, sectionData);
         if (Object.keys(sectionDelta).length > 0) {
           await apiClient.put(`/api/v1/admin/settings/section/${sectionName}`, sectionDelta);
         }
 
-        // Save delta settings if provided
+        // Save delta settings if provided (filter to only changed values)
         if (deltaSettings && Object.keys(deltaSettings).length > 0) {
-          await apiClient.put('/api/v1/admin/settings', { settings: deltaSettings });
+          // Build deltaSettings from original using same transformer to get correct structure
+          const { deltaSettings: originalDeltaSettings } = saveTransformer(originalSettings);
+
+          console.log(`[useAdminSettings:${sectionName}] Comparing deltaSettings:`, {
+            original: originalDeltaSettings,
+            current: deltaSettings
+          });
+
+          // Compare current vs original deltaSettings (both have same backend paths)
+          const changedDeltaSettings: Record<string, any> = {};
+          for (const [key, value] of Object.entries(deltaSettings)) {
+            const originalValue = originalDeltaSettings?.[key];
+
+            // Only include if value actually changed
+            if (JSON.stringify(value) !== JSON.stringify(originalValue)) {
+              changedDeltaSettings[key] = value;
+              console.log(`[useAdminSettings:${sectionName}] Delta field changed: ${key}`, {
+                original: originalValue,
+                new: value
+              });
+            }
+          }
+
+          if (Object.keys(changedDeltaSettings).length > 0) {
+            console.log(`[useAdminSettings:${sectionName}] Sending delta settings:`, changedDeltaSettings);
+            await apiClient.put('/api/v1/admin/settings', { settings: changedDeltaSettings });
+          } else {
+            console.log(`[useAdminSettings:${sectionName}] No delta settings changed, skipping`);
+          }
         }
       } else {
         // Simple single-endpoint save with delta

frontend/src/core/hooks/useAuditFilters.ts

@@ -0,0 +1,59 @@
+import { useState, useEffect } from 'react';
+import auditService, { AuditFilters } from '@app/services/auditService';
+
+/**
+ * Shared hook for managing audit filters across components
+ */
+export function useAuditFilters(initialFilters: Partial<AuditFilters> = {}) {
+  const [eventTypes, setEventTypes] = useState<string[]>([]);
+  const [users, setUsers] = useState<string[]>([]);
+  const [filters, setFilters] = useState<AuditFilters>({
+    eventType: undefined,
+    username: undefined,
+    startDate: undefined,
+    endDate: undefined,
+    ...initialFilters,
+  });
+
+  // Fetch metadata on mount
+  useEffect(() => {
+    const fetchMetadata = async () => {
+      try {
+        const [types, usersList] = await Promise.all([
+          auditService.getEventTypes(),
+          auditService.getUsers(),
+        ]);
+        setEventTypes(types);
+        setUsers(usersList);
+      } catch (err) {
+        console.error('Failed to fetch audit metadata:', err);
+      }
+    };
+
+    fetchMetadata();
+  }, []);
+
+  const handleFilterChange = (key: keyof AuditFilters, value: any) => {
+    setFilters((prev) => ({ ...prev, [key]: value }));
+  };
+
+  const handleClearFilters = () => {
+    setFilters({
+      eventType: undefined,
+      username: undefined,
+      startDate: undefined,
+      endDate: undefined,
+      page: initialFilters.page,
+      pageSize: initialFilters.pageSize,
+    });
+  };
+
+  return {
+    filters,
+    setFilters,
+    eventTypes,
+    users,
+    handleFilterChange,
+    handleClearFilters,
+  };
+}