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/components/shared/requirement-upload-slot.tsx

@@ -72,7 +72,7 @@ interface RequirementUploadSlotProps {
 
 function ViewFileButton({ bucket, objectKey }: { bucket: string; objectKey: string }) {
   const { data } = trpc.file.getDownloadUrl.useQuery(
-    { bucket, objectKey, forDownload: false },
+    { bucket, objectKey, forDownload: false, purpose: 'open' as const },
     { staleTime: 10 * 60 * 1000 }
   )
   const href = typeof data === 'string' ? data : data?.url
@@ -87,7 +87,7 @@ function ViewFileButton({ bucket, objectKey }: { bucket: string; objectKey: stri
 
 function DownloadFileButton({ bucket, objectKey, fileName }: { bucket: string; objectKey: string; fileName: string }) {
   const { data } = trpc.file.getDownloadUrl.useQuery(
-    { bucket, objectKey, forDownload: true, fileName },
+    { bucket, objectKey, forDownload: true, fileName, purpose: 'download' as const },
     { staleTime: 10 * 60 * 1000 }
   )
   const href = typeof data === 'string' ? data : data?.url
@@ -229,7 +229,7 @@ export function RequirementUploadSlot({
 
   // Fetch preview URL only when preview is toggled on
   const { data: previewUrlData, isLoading: isLoadingPreview } = trpc.file.getDownloadUrl.useQuery(
-    { bucket: existingFile?.bucket || '', objectKey: existingFile?.objectKey || '', forDownload: false },
+    { bucket: existingFile?.bucket || '', objectKey: existingFile?.objectKey || '', forDownload: false, purpose: 'preview' as const },
     { enabled: showPreview && !!existingFile?.bucket && !!existingFile?.objectKey, staleTime: 10 * 60 * 1000 }
   )
   const previewUrl = typeof previewUrlData === 'string' ? previewUrlData : previewUrlData?.url