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/router.go

@@ -141,6 +141,7 @@ func NewRouter(
 		pools.GET("/:pool_id", poolHandler.Get)
 		pools.PATCH("/:pool_id", poolHandler.Update)
 		pools.DELETE("/:pool_id", poolHandler.Delete)
+		pools.POST("/:pool_id/views", poolHandler.RecordView)
 
 		// Sub-routes registered before /:pool_id/files to avoid param conflicts.
 		pools.POST("/:pool_id/files/remove", poolHandler.RemoveFiles)