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

backend/src/db/schema.ts

@@ -365,6 +365,8 @@ export const uptimeProbeSamples = pgTable("uptime_probe_samples", {
   rttMs:     integer("rtt_ms"),
   lossPct:   integer("loss_pct").notNull().default(0),
   status:    text("status", { enum: ["up", "warn", "down"] }).notNull().default("down"),
+  measurementKnown: boolean("measurement_known").notNull().default(true),
+  observationError: text("observation_error"),
 }, (t) => [
   primaryKey({ columns: [t.probeId, t.sampledAt] }),
 ])