fix(task): prevent duplicate Slurm job submission on backend restart

RecoverStuckTasks now skips tasks that already have a slurm_job_id,
and ProcessTask adds a guard before the submitting step to prevent
re-submission even if a task is incorrectly re-enqueued.

Also deprecates POST /api/v1/jobs/submit endpoint (replaced by POST /tasks)
and comments out related handlers and tests.

internal/server/server_test.go

@@ -17,7 +17,7 @@ func TestAllRoutesRegistered(t *testing.T) {
 		method string
 		path   string
 	}{
-		{"POST", "/api/v1/jobs/submit"},
+		// {"POST", "/api/v1/jobs/submit"}, // [已弃用] 已被 POST /tasks 取代
 		{"GET", "/api/v1/jobs"},
 		{"GET", "/api/v1/jobs/history"},
 		{"GET", "/api/v1/jobs/:id"},