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>

backend/internal/handler/router.go

@@ -84,6 +84,7 @@ func NewRouter(
 		files.PUT("/:id/content", fileHandler.ReplaceContent)
 		files.GET("/:id/thumbnail", fileHandler.GetThumbnail)
 		files.GET("/:id/preview", fileHandler.GetPreview)
+		files.POST("/:id/views", fileHandler.RecordView)
 		files.POST("/:id/restore", fileHandler.Restore)
 		files.DELETE("/:id/permanent", fileHandler.PermanentDelete)