feat(backend): record pool views to activity.pool_views

Add POST /pools/{id}/views, mirroring the file-view endpoint: it
enforces view ACL and appends a row to activity.pool_views (viewed_at
defaults to statement_timestamp(), so each view is its own history row).
The table existed but nothing wrote to it.

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

backend/internal/handler/pool_handler.go

@@ -160,6 +160,25 @@ func (h *PoolHandler) Get(c *gin.Context) {
 	respondJSON(c, http.StatusOK, toPoolJSON(*p))
 }
 
+// ---------------------------------------------------------------------------
+// POST /pools/:pool_id/views
+// ---------------------------------------------------------------------------
+
+// RecordView logs that the current user viewed the pool (activity.pool_views).
+func (h *PoolHandler) RecordView(c *gin.Context) {
+	id, ok := parsePoolID(c)
+	if !ok {
+		return
+	}
+
+	if err := h.poolSvc.RecordView(c.Request.Context(), id); err != nil {
+		respondError(c, err)
+		return
+	}
+
+	c.Status(http.StatusNoContent)
+}
+
 // ---------------------------------------------------------------------------
 // PATCH /pools/:pool_id
 // ---------------------------------------------------------------------------