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).

main.go

@@ -53,10 +53,8 @@ func runWebServer() {
 
 	godotenv.Load()
 
-	if limit, source := sys.ApplyMemoryLimit(); limit > 0 {
-		logger.Infof("Go memory soft limit set to %d MiB (%s)", limit>>20, source)
-	} else {
-		logger.Info("Go memory soft limit not enforced: ", source)
+	for _, line := range sys.ApplyMemoryTuning() {
+		logger.Info(line)
 	}
 
 	if os.Getenv("XUI_PPROF") == "true" {