test(backend): enhance stubs with logging, heartbeat, timed exit, and fake downloads

arma3server_x64 stub:
- Logs binary path and all arguments ([STUB] binary=... args=...)
- Prints heartbeat every 5 seconds
- Accepts -t N flag to exit after N seconds (for auto-exit tests)

steamcmd stub:
- Parses +force_install_dir and +workshop_download_item args
- Creates fake mod directories at the expected workshop content path
- Prints fake download progress and success logs

New tests:
- TestStart_StubLogsArgsAndPath: verifies arg logging via log streamer
- TestStart_StubLogsBinaryPath: verifies binary path logging
- TestStart_StubHeartbeatLogs: verifies heartbeat lines appear within timeout
- TestStart_StubAutoExit: verifies -t flag causes server to auto-exit
- TestDownloadMods_StubCreatesModDirs: verifies single mod dir creation
- TestDownloadMods_StubCreatesMultipleModDirs: verifies multi-mod dir creation
- TestDownloadMods_StubLogsAppear: verifies fake download logs on channel
- TestDownloadMods_StubSteamcmdArgsLogged: verifies install_dir logging

backend/internal/services/testdata/arma3server_x64

@@ -1,5 +1,31 @@
 #!/bin/sh
 # Stub arma3server binary for testing.
-# Logs args to stdout then sleeps until killed (simulates a running server).
-echo "[STUB] arma3server_x64 called with: $@"
-sleep 3600
+# Logs binary path and all arguments, prints heartbeat every 5s.
+# Use -t N to exit after N seconds (for auto-exit tests).
+echo "[STUB] binary=$0 args=$*"
+
+DURATION=0
+while [ $# -gt 0 ]; do
+  case "$1" in
+    -t) DURATION="$2"; shift 2 ;;
+    *)  shift ;;
+  esac
+done
+
+if [ "$DURATION" -gt 0 ] 2>/dev/null; then
+  ELAPSED=0
+  while [ "$ELAPSED" -lt "$DURATION" ]; do
+    REMAINING=$((DURATION - ELAPSED))
+    WAIT=5
+    if [ "$REMAINING" -lt 5 ]; then WAIT=$REMAINING; fi
+    sleep "$WAIT"
+    ELAPSED=$((ELAPSED + WAIT))
+    echo "[STUB] heartbeat elapsed=${ELAPSED}s remaining=$((REMAINING - WAIT))s"
+  done
+  echo "[STUB] server exiting after ${DURATION}s"
+else
+  while true; do
+    sleep 5
+    echo "[STUB] heartbeat running..."
+  done
+fi