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/audit/page.tsx

@@ -77,6 +77,8 @@ const ACTION_TYPES = [
   'ROUND_ARCHIVED',
   'UPLOAD_FILE',
   'DELETE_FILE',
+  'FILE_VIEWED',
+  'FILE_OPENED',
   'FILE_DOWNLOADED',
   'BULK_CREATE',
   'BULK_UPDATE_STATUS',
@@ -171,6 +173,8 @@ const actionColors: Record<string, 'default' | 'destructive' | 'secondary' | 'ou
   ROUND_ACTIVATED: 'default',
   ROUND_CLOSED: 'secondary',
   ROUND_ARCHIVED: 'secondary',
+  FILE_VIEWED: 'outline',
+  FILE_OPENED: 'outline',
   FILE_DOWNLOADED: 'outline',
   ROLE_CHANGED: 'secondary',
   PASSWORD_SET: 'outline',