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/file_handler.go

@@ -300,6 +300,25 @@ func (h *FileHandler) GetMeta(c *gin.Context) {
 	respondJSON(c, http.StatusOK, toFileJSON(*f))
 }
 
+// ---------------------------------------------------------------------------
+// POST /files/:id/views
+// ---------------------------------------------------------------------------
+
+// RecordView logs that the current user viewed the file (activity.file_views).
+func (h *FileHandler) RecordView(c *gin.Context) {
+	id, ok := parseFileID(c)
+	if !ok {
+		return
+	}
+
+	if err := h.fileSvc.RecordView(c.Request.Context(), id); err != nil {
+		respondError(c, err)
+		return
+	}
+
+	c.Status(http.StatusNoContent)
+}
+
 // ---------------------------------------------------------------------------
 // PATCH /files/:id
 // ---------------------------------------------------------------------------