refactor: represent time fields as int64 Unix seconds throughout the stack

Collapses a four-step conversion chain (time_t → int64 → time.Time →
RFC3339 string → string → time.Time) to a direct int64 pass-through:

  C time_t → rawJobData int64 → JobInfo int64 → JSON integer → QueryJob int64 → time.Unix() for display

Changes:
- apiserver/models/jobs.go: SubmitTime, QueueTime, StartTime, EndTime,
  ReserveTime in JobInfo and JobAdditionalFields: time.Time → int64
- apiserver/models/common.go: ActionControl.SubmitTime: time.Time → int64
- backend/convert_pure.go: remove time.Unix() calls; assign int64 directly
- backend/convert_test.go: update time assertions to compare int64 values
- xcecli/models/jobs.go: SubmitTime, StartTime, EndTime, ReserveTime in
  QueryJob: string/time.Time → int64; remove time import
- xcecli/common/output/time.go: replace parseJobTime(string) with
  unixToTime(int64) and fmtUnixTime(int64) helpers
- xcecli/common/output/page_print_jobs.go: replace parseJobTime() calls
  with unixToTime()/fmtUnixTime(); remove !startTime.IsZero() guard
  (unixToTime returns false for ts<=0, making it redundant)
- xcecli/common/output/page_print_array_elements_jobs.go: replace
  string-split+time.Parse submit-time block with fmtUnixTime(); remove
  unused time import

Side effect: eliminates the "0001-01-01T00:00:00Z" zero-time
serialization issue at the source — zero timestamps are now 0 (integer)
and unixToTime/fmtUnixTime return false/"-" for ts<=0 without special-
casing needed in callers.

src/frontend/xcecli/common/output/time.go

@@ -5,17 +5,23 @@ import (
 	"time"
 )
 
-// parseJobTime parses a job time string supporting RFC3339 and space-separated formats.
-func parseJobTime(s string) (time.Time, error) {
-	if s == "" {
-		return time.Time{}, fmt.Errorf("empty time string")
+// unixToTime converts a Unix-second timestamp to time.Time.
+// Returns (time, true) for ts > 0; (zero, false) for unset/zero values.
+func unixToTime(ts int64) (time.Time, bool) {
+	if ts <= 0 {
+		return time.Time{}, false
 	}
-	for _, f := range []string{time.RFC3339Nano, time.RFC3339, "2006-01-02 15:04:05", "2006-01-02 15:04:05 MST"} {
-		if t, err := time.Parse(f, s); err == nil {
-			return t, nil
-		}
+	return time.Unix(ts, 0), true
+}
+
+// fmtUnixTime formats a Unix timestamp as "Jan 02 15:04" for table display.
+// Returns "-" for zero/unset values.
+func fmtUnixTime(ts int64) string {
+	t, ok := unixToTime(ts)
+	if !ok {
+		return "-"
 	}
-	return time.Time{}, fmt.Errorf("cannot parse time: %q", s)
+	return formatJobTime(t)
 }
 
 // formatJobTime formats a time.Time as "Jan 02 15:04" for table display.