fix(uptime): отличать ошибки опроса от измеренных потерь Ping

backend/drizzle/0010_ping_measurement_known.sql

@@ -0,0 +1,3 @@
+-- A failed command is an attempt, not a measured packet-loss percentage.
+ALTER TABLE uptime_probe_samples ADD COLUMN IF NOT EXISTS measurement_known BOOLEAN NOT NULL DEFAULT true;
+ALTER TABLE uptime_probe_samples ADD COLUMN IF NOT EXISTS observation_error TEXT;