cmd/speedtest-sync: fail-fast on bad timestamps + minor cleanups

- Skip rows with null/malformed timestamps instead of silently
  re-stamping with time.Now() (data corruption hidden behind a
  fallback). Drops the time import.
- Replace ad-hoc parentDir() with stdlib filepath.Dir().
- Add t.Fatal error checks to TestExtractRows_SkipsBadExtra
  (CREATE TABLE / INSERT) and switch sql.Open to checked form.
- New TestExtractRows_SkipsBadTimestamp covers the null-ts path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/speedtest-sync/sync.go

@@ -6,7 +6,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
-	"time"
 
 	pb "github.com/bergabruh/stats-gateway/gen/stats/v1"
 )
@@ -63,12 +62,14 @@ func extractRows(ctx context.Context, db *sql.DB, sinceID int64) ([]extractedRow
 			continue
 		}
 
-		var tsUnix int64
-		if ts.Valid {
-			fmt.Sscanf(ts.String, "%d", &tsUnix)
+		if !ts.Valid {
+			log.Printf("skip id=%d: null timestamp", id)
+			continue
 		}
-		if tsUnix == 0 {
-			tsUnix = time.Now().Unix()
+		var tsUnix int64
+		if _, err := fmt.Sscanf(ts.String, "%d", &tsUnix); err != nil || tsUnix == 0 {
+			log.Printf("skip id=%d: bad timestamp %q", id, ts.String)
+			continue
 		}
 
 		out = append(out, extractedRow{