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/apiserver/models/common.go

@@ -1,7 +1,5 @@
 package models
 
-import "time"
-
 const (
 	CONTROL_JOB     = 1
 	CONTROL_HOST    = 2
@@ -11,9 +9,9 @@ const (
 )
 
 type ActionControl struct {
-	TargetType int       `json:"target_type"`
-	ActionType int       `json:"action_type"`
-	Content    Jsonb     `json:"content"`
-	SubmitTime time.Time `json:"submit_time"`
-	Submitter  string    `json:"submitter"`
+	TargetType int    `json:"target_type"`
+	ActionType int    `json:"action_type"`
+	Content    Jsonb  `json:"content"`
+	SubmitTime int64  `json:"submit_time"`
+	Submitter  string `json:"submitter"`
 }

src/frontend/apiserver/models/jobs.go

@@ -489,7 +489,7 @@ type JobInfo struct {
 	ArrayIdx           int        `json:"array_idx"` //new
 	SubmitUser         string     `json:"submit_user"`
 	FromHost           string     `json:"from_host"`
-	SubmitTime         time.Time  `json:"submit_time"`
+	SubmitTime         int64      `json:"submit_time"`
 	JobStatus          int        `json:"job_status"`
 	JobName            string     `json:"job_name"`
 	Command            string     `json:"command"`
@@ -535,10 +535,10 @@ type JobInfo struct {
 	Tags               Jsonb      `json:"tags"`                 // new, JSONB as []byte
 	EffectiveResReq    string     `json:"effective_resreq"`     //new
 	EffectiveUserGroup string     `json:"effective_user_group"` //new
-	QueueTime          time.Time  `json:"queue_time"`
-	StartTime          time.Time  `json:"start_time"`
-	EndTime            time.Time  `json:"end_time"`
-	ReserveTime        time.Time  `json:"reserve_time"`
+	QueueTime          int64      `json:"queue_time"`
+	StartTime          int64      `json:"start_time"`
+	EndTime            int64      `json:"end_time"`
+	ReserveTime        int64      `json:"reserve_time"`
 	Duration           int        `json:"duration"`
 	CPUTime            int        `json:"cpu_time"`
 	ExitStatus         Jsonb      `json:"exit_status"` //change type,  JSONB as []byte
@@ -589,12 +589,12 @@ type JobAdditionalFields struct {
 	JobID      int64     `json:"job_id"`
 	SubmitUser string    `json:"submit_user"`
 	JobStatus  int       `json:"job_status"`
-	SubmitTime time.Time `json:"submit_time"`
-	FromHost   string    `json:"from_host"`
-	ExecHosts  string    `json:"exec_hosts"`
+	SubmitTime int64  `json:"submit_time"`
+	FromHost   string `json:"from_host"`
+	ExecHosts  string `json:"exec_hosts"`
 
-	StartTime         time.Time `json:"start_time"`
-	EndTime           time.Time `json:"end_time"`
+	StartTime         int64 `json:"start_time"`
+	EndTime           int64 `json:"end_time"`
 	JobRusage         Jsonb     `json:"job_rusage"`
 	MaxMem            float64   `json:"max_mem"`
 	AvgMem            float64   `json:"avg_mem"`