Auto-pause physics engine when no one is viewing

- Physics pauses after IDLE_PAUSE_SECONDS (default 30s) of inactivity
- Resumes instantly when someone views the dashboard
- No container restart needed - just pauses the simulation loop
- CPU usage drops to ~0% when paused

src/py_dvt_ate/app/dashboard/app.py

@@ -8,7 +8,6 @@ thermal-electrical coupling in real-time using instrument interfaces.
 import asyncio
 import atexit
 import os
-import sys
 import threading
 import time
 from collections import deque
@@ -29,40 +28,36 @@ from py_dvt_ate.tests.thermal.tempco import TempCoTest
 # Thread pool for background test execution
 _test_executor = ThreadPoolExecutor(max_workers=1, thread_name_prefix="test_runner")
 
-# Idle shutdown configuration
-# Set IDLE_TIMEOUT_MINUTES=0 to disable auto-shutdown
-IDLE_TIMEOUT_MINUTES = int(os.environ.get("IDLE_TIMEOUT_MINUTES", "0"))
+# Idle pause configuration
+# Physics engine pauses after IDLE_PAUSE_SECONDS of no activity (default: 30s)
+IDLE_PAUSE_SECONDS = int(os.environ.get("IDLE_PAUSE_SECONDS", "30"))
 _last_activity_time: float = time.time()
-_idle_monitor_started = False
+_idle_checker_started = False
+_server_ref: SimulationServer | None = None  # Reference for idle checker thread
 
 
-def _update_activity() -> None:
-    """Update the last activity timestamp."""
-    global _last_activity_time
-    _last_activity_time = time.time()
-
-
-def _idle_monitor() -> None:
-    """Background thread that exits the app after idle timeout."""
-    global _last_activity_time
+def _idle_checker() -> None:
+    """Background thread that pauses physics when idle."""
+    global _last_activity_time, _server_ref
     while True:
-        time.sleep(60)  # Check every minute
-        if IDLE_TIMEOUT_MINUTES <= 0:
+        time.sleep(5)  # Check every 5 seconds
+        if _server_ref is None:
             continue
-        idle_minutes = (time.time() - _last_activity_time) / 60
-        if idle_minutes >= IDLE_TIMEOUT_MINUTES:
-            print(f"Idle timeout reached ({IDLE_TIMEOUT_MINUTES} minutes). Shutting down.")
-            os._exit(0)
+
+        idle_seconds = time.time() - _last_activity_time
+        if idle_seconds > IDLE_PAUSE_SECONDS and not _server_ref.paused:
+            _server_ref.paused = True
+            print(f"Physics engine paused (idle for {idle_seconds:.0f}s)")
 
 
-def _start_idle_monitor() -> None:
-    """Start the idle monitor thread if timeout is configured."""
-    global _idle_monitor_started
-    if IDLE_TIMEOUT_MINUTES > 0 and not _idle_monitor_started:
-        _idle_monitor_started = True
-        thread = threading.Thread(target=_idle_monitor, daemon=True)
+def _start_idle_checker(server: SimulationServer) -> None:
+    """Start the idle checker thread."""
+    global _idle_checker_started, _server_ref
+    _server_ref = server
+    if not _idle_checker_started:
+        _idle_checker_started = True
+        thread = threading.Thread(target=_idle_checker, daemon=True)
         thread.start()
-        print(f"Idle auto-shutdown enabled: {IDLE_TIMEOUT_MINUTES} minutes")
 
 # History buffer size for charts
 HISTORY_SIZE = 500
@@ -159,12 +154,21 @@ def start_embedded_server() -> tuple[SimulationServer, threading.Thread]:
     return server, thread
 
 
+def _update_activity() -> None:
+    """Update activity timestamp and resume physics if paused."""
+    global _last_activity_time
+    _last_activity_time = time.time()
+
+    # Resume physics if it was paused
+    if "server" in st.session_state:
+        server = st.session_state.server
+        if server.paused:
+            server.paused = False
+            print("Physics engine resumed (user activity detected)")
+
+
 def init_session_state() -> None:
     """Initialise Streamlit session state."""
-    # Start idle monitor for auto-shutdown
-    _start_idle_monitor()
-    _update_activity()
-
     if "server" not in st.session_state:
         with st.spinner("Starting simulation server..."):
             st.session_state.server, st.session_state.server_thread = start_embedded_server()
@@ -174,6 +178,9 @@ def init_session_state() -> None:
             st.error("Failed to start simulation server. Please refresh the page.")
             st.stop()
 
+        # Start idle checker to pause physics when no one's viewing
+        _start_idle_checker(st.session_state.server)
+
         # Register cleanup
         def cleanup() -> None:
             if hasattr(st.session_state, "server") and st.session_state.server is not None:
@@ -427,7 +434,7 @@ def display_controls() -> None:
 @st.fragment(run_every=0.1)
 def simulation_display() -> None:
     """Fragment that displays and updates simulation state."""
-    _update_activity()  # Track that someone is viewing the dashboard
+    _update_activity()  # Track activity and resume physics if paused
 
     if "server" not in st.session_state:
         st.warning("Initializing simulation server...")