perf(memory): report real RSS and cut footprint via GOGC + periodic release

The Usage card showed runtime.MemStats.Sys, a never-shrinking high-water mark of reserved address space that also counts memory already returned to the OS, so it overstated real usage (e.g. ~300 MB on an idle 1-client server). Report process RSS instead so the number matches the OS and drops as memory is freed.

Replace the auto GOMEMLIMIT that targeted ~90 percent of total system RAM (a near no-op while the heap sits far below the limit, and a GC-thrash risk on small/shared VPS per go.dev/doc/gc-guide) with: a lower default GOGC (XUI_GOGC, default 75), a periodic debug.FreeOSMemory job (XUI_MEMORY_RELEASE_INTERVAL, default 10m, 0 disables), and a soft limit applied only from an explicit budget (GOMEMLIMIT, XUI_MEMORY_LIMIT, or a real cgroup cap at 90 percent).

internal/web/web.go

@@ -21,6 +21,7 @@ import (
 	"github.com/mhsanaei/3x-ui/v3/internal/logger"
 	"github.com/mhsanaei/3x-ui/v3/internal/mtproto"
 	"github.com/mhsanaei/3x-ui/v3/internal/util/common"
+	"github.com/mhsanaei/3x-ui/v3/internal/util/sys"
 	"github.com/mhsanaei/3x-ui/v3/internal/web/controller"
 	"github.com/mhsanaei/3x-ui/v3/internal/web/job"
 	"github.com/mhsanaei/3x-ui/v3/internal/web/locale"
@@ -394,6 +395,10 @@ func (s *Server) startTask(restartXray bool) {
 	if s.memoryAlarmWanted() {
 		s.cron.AddJob(cadenceMemoryAlarm, job.NewCheckMemJob())
 	}
+
+	if mins := sys.MemoryReleaseIntervalMinutes(); mins > 0 {
+		s.cron.AddJob(fmt.Sprintf("@every %dm", mins), job.NewMemoryReleaseJob())
+	}
 }
 
 // cpuAlarmWanted reports whether any notifier is configured to receive cpu.high