feat(backend): log file views

The activity.file_views table existed but nothing ever wrote to it. Add a
POST /files/{id}/views endpoint: FileRepo.RecordView inserts a history row,
FileService.RecordView enforces view ACL first. The file viewer fires it
(fire-and-forget) when a file is opened, including while paging prev/next.

Documented in openapi.yaml; covered by TestRecordFileView (204 on view,
repeatable, 404 for unknown file).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/components/file/FileViewer.svelte

@@ -77,6 +77,9 @@
 			isPublic = fileData.is_public ?? false;
 			dirty = false;
 			void fetchPreview(id);
+			// Log the view (activity.file_views). Fire-and-forget — never block or
+			// fail the viewer over view tracking.
+			void api.post(`/files/${id}/views`).catch(() => {});
 		} catch (e) {
 			error = e instanceof ApiError ? e.message : 'Failed to load file';
 		} finally {