feat: granular file access audit logging (viewed/opened/downloaded)

Replace single FILE_DOWNLOADED action with three granular actions:
- FILE_VIEWED: inline preview loaded in the UI
- FILE_OPENED: file opened in a new browser tab
- FILE_DOWNLOADED: explicit download button clicked

Add 'purpose' field to getDownloadUrl input (preview/open/download).
All client callers updated to pass the appropriate purpose.
Audit page updated with new filter options and color mappings.

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

src/app/(applicant)/applicant/documents/page.tsx

@@ -46,11 +46,11 @@ const fileTypeLabels: Record<string, string> = {
 
 function FileActionButtons({ bucket, objectKey, fileName }: { bucket: string; objectKey: string; fileName: string }) {
   const { data: viewData } = trpc.file.getDownloadUrl.useQuery(
-    { bucket, objectKey, forDownload: false },
+    { bucket, objectKey, forDownload: false, purpose: 'open' as const },
     { staleTime: 10 * 60 * 1000 }
   )
   const { data: dlData } = trpc.file.getDownloadUrl.useQuery(
-    { bucket, objectKey, forDownload: true, fileName },
+    { bucket, objectKey, forDownload: true, fileName, purpose: 'download' as const },
     { staleTime: 10 * 60 * 1000 }
   )
   const viewUrl = typeof viewData === 'string' ? viewData : viewData?.url