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/models/jobs.go

@@ -3,8 +3,6 @@ package models
 import (
 	"encoding/json"
 	"fmt"
-	"time"
-	
 )
 
 const (
@@ -115,10 +113,10 @@ type QueryJob struct {
 	TermTime           string        `json:"term_time"`
 	UserPriority       int           `json:"user_priority"`
 	DependCond         string        `json:"depend_cond"`
-	SubmitTime         string        `json:"submit_time"`
-	ReserveTime        time.Time     `json:"reserve_time"`
-	StartTime          string        `json:"start_time"`
-	EndTime            string        `json:"end_time"`
+	SubmitTime         int64         `json:"submit_time"`
+	ReserveTime        int64         `json:"reserve_time"`
+	StartTime          int64         `json:"start_time"`
+	EndTime            int64         `json:"end_time"`
 	JobRusage          string        `json:"job_rusage"`
 	MaxMem             float64       `json:"max_mem"`
 	AvgMem             float64       `json:"avg_mem"`