fix: 503 при сбое GetScan вместо ложного 404 scan not found

- Store.GetScan возвращает (job, ok, error); Postgres отдаёт err при ошибке БД/JSON

- API: getScanOrWriteError → 503 scan storage temporarily unavailable

- Web UI: понятное сообщение для HTTP 503 при опросе скана

Made-with: Cursor

internal/scans/postgres_store.go

@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"log"
 	"sort"
 	"strings"
@@ -98,7 +99,7 @@ insert into scan_jobs (
 	return job, nil
 }
 
-func (s *PostgresStore) GetScan(id string) (ScanJob, bool) {
+func (s *PostgresStore) GetScan(id string) (ScanJob, bool, error) {
 	query := `
 select id, name, status, cidrs::text, exclude_ips::text, options::text,
        progress, stats::text, coalesce(snmp_credentials_id::text, ''),
@@ -130,28 +131,28 @@ limit 1`
 		&finishedAt,
 	)
 	if errors.Is(err, sql.ErrNoRows) {
-		return ScanJob{}, false
+		return ScanJob{}, false, nil
 	}
 	if err != nil {
 		log.Printf("postgres GetScan id=%q: row scan: %v", id, err)
-		return ScanJob{}, false
+		return ScanJob{}, false, fmt.Errorf("postgres GetScan id=%q: %w", id, err)
 	}
 
 	if err = json.Unmarshal([]byte(cidrsRaw), &job.CIDRs); err != nil {
 		log.Printf("postgres GetScan id=%q: cidrs json (len=%d): %v", id, len(cidrsRaw), err)
-		return ScanJob{}, false
+		return ScanJob{}, false, fmt.Errorf("postgres GetScan id=%q cidrs: %w", id, err)
 	}
 	if err = json.Unmarshal([]byte(excludeRaw), &job.ExcludeIPs); err != nil {
 		log.Printf("postgres GetScan id=%q: exclude_ips json: %v", id, err)
-		return ScanJob{}, false
+		return ScanJob{}, false, fmt.Errorf("postgres GetScan id=%q exclude_ips: %w", id, err)
 	}
 	if err = json.Unmarshal([]byte(optionsRaw), &job.Options); err != nil {
 		log.Printf("postgres GetScan id=%q: options json: %v", id, err)
-		return ScanJob{}, false
+		return ScanJob{}, false, fmt.Errorf("postgres GetScan id=%q options: %w", id, err)
 	}
 	if err = json.Unmarshal([]byte(statsRaw), &job.Stats); err != nil {
 		log.Printf("postgres GetScan id=%q: stats json: %v", id, err)
-		return ScanJob{}, false
+		return ScanJob{}, false, fmt.Errorf("postgres GetScan id=%q stats: %w", id, err)
 	}
 	if startedAt.Valid {
 		job.StartedAt = startedAt.Time
@@ -160,7 +161,7 @@ limit 1`
 		job.FinishedAt = finishedAt.Time
 	}
 
-	return job, true
+	return job, true, nil
 }
 
 func (s *PostgresStore) ListScans(limit int) []ScanJob {
@@ -230,7 +231,11 @@ limit $1`
 }
 
 func (s *PostgresStore) UpdateScan(id string, update ScanUpdate) (ScanJob, bool) {
-	current, ok := s.GetScan(id)
+	current, ok, err := s.GetScan(id)
+	if err != nil {
+		log.Printf("postgres UpdateScan: GetScan id=%q: %v", id, err)
+		return ScanJob{}, false
+	}
 	if !ok {
 		return ScanJob{}, false
 	}