perf: remote browser CPU / zombie optimization

- Add tini (init: true in compose) to reap orphan Chromium processes
- Reduce screenshot push frequency (active 0.12→0.20s, idle 0.35→1.00s,
  deep idle 1.00→5.00s, backoff 0.60→2.00s)
- Add 5s timeout to screenshot in WebSocket push loop
- close() now wraps context.close() in asyncio.wait_for(10s)
  with browser.close() fallback on timeout
- Two-phase close logging (closing → closed / close_failed)
- Auth-capture sessions evicted after 10min TTL
- shutdown() with timeout protection and logging
- close_ok correctly tracks success through browser fallback path

backend/app/routers/browser_sessions.py

@@ -265,12 +265,12 @@ async def clear_profile(custom_page_id: int, _=Depends(get_current_user)):
 
 
 # ——— WebSocket stream ———
-# Frame interval & diff detection
-_WS_MIN_INTERVAL = 0.10
-_WS_IDLE_INTERVAL = 0.35
-_WS_ACTIVE_INTERVAL = 0.12
-_WS_BACKOFF_INTERVAL = 0.60
-_WS_DEEP_IDLE_INTERVAL = 1.00
+# Frame interval & diff detection (tuned for CPU efficiency)
+_WS_MIN_INTERVAL = 0.15
+_WS_IDLE_INTERVAL = 1.00
+_WS_ACTIVE_INTERVAL = 0.20
+_WS_BACKOFF_INTERVAL = 2.00
+_WS_DEEP_IDLE_INTERVAL = 5.00
 _WS_ACTIVE_WINDOW = 1.25
 
 
@@ -361,7 +361,12 @@ async def session_ws(
                         state = await browser_sessions.state(session_id)
                         await websocket.send_json({"type": "state", "session": state})
 
-                    frame = await browser_sessions.screenshot(session_id)
+                    frame = await asyncio.wait_for(
+                        browser_sessions.screenshot(session_id), timeout=5.0)
+                except asyncio.TimeoutError:
+                    logger.warning("ws screenshot timeout for %s", session_id[:12])
+                    await asyncio.sleep(interval)
+                    continue
                 except KeyError:
                     await websocket.send_json({"error": "session_not_found"})
                     break