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/(admin)/admin/projects/bulk-upload/page.tsx

@@ -146,7 +146,7 @@ export default function BulkUploadPage() {
   const handleViewFile = useCallback(
     async (bucket: string, objectKey: string) => {
       try {
-        const { url } = await utils.file.getDownloadUrl.fetch({ bucket, objectKey })
+        const { url } = await utils.file.getDownloadUrl.fetch({ bucket, objectKey, purpose: 'open' as const })
         window.open(url, '_blank')
       } catch {
         toast.error('Failed to open file. It may have been deleted from storage.')