refactor(server): migrate to pool-based crawler management

Replace crawler_manager.py with simpler crawler_pool.py implementation:
- Add global page semaphore for hard concurrency cap
- Implement browser pool with idle cleanup
- Add playground UI for testing and stress testing
- Update API handlers to use pooled crawlers
- Enhance logging levels and symbols

BREAKING CHANGE: Removes CrawlerManager class in favor of simpler pool-based approach

deploy/docker/crawler_pool.py

@@ -0,0 +1,60 @@
+# crawler_pool.py  (new file)
+import asyncio, json, hashlib, time, psutil
+from contextlib import suppress
+from typing import Dict
+from crawl4ai import AsyncWebCrawler, BrowserConfig
+from typing import Dict
+from utils import load_config 
+
+CONFIG = load_config()
+
+POOL: Dict[str, AsyncWebCrawler] = {}
+LAST_USED: Dict[str, float] = {}
+LOCK = asyncio.Lock()
+
+MEM_LIMIT  = CONFIG.get("crawler", {}).get("memory_threshold_percent", 95.0)   # % RAM – refuse new browsers above this
+IDLE_TTL  = CONFIG.get("crawler", {}).get("pool", {}).get("idle_ttl_sec", 1800)   # close if unused for 30 min
+
+def _sig(cfg: BrowserConfig) -> str:
+    payload = json.dumps(cfg.to_dict(), sort_keys=True, separators=(",",":"))
+    return hashlib.sha1(payload.encode()).hexdigest()
+
+async def get_crawler(cfg: BrowserConfig) -> AsyncWebCrawler:
+    try:
+        sig = _sig(cfg)
+        async with LOCK:
+            if sig in POOL:
+                LAST_USED[sig] = time.time();  
+                return POOL[sig]
+            if psutil.virtual_memory().percent >= MEM_LIMIT:
+                raise MemoryError("RAM pressure – new browser denied")
+            crawler = AsyncWebCrawler(config=cfg, thread_safe=False)
+            await crawler.start()
+            POOL[sig] = crawler; LAST_USED[sig] = time.time()
+            return crawler
+    except MemoryError as e:
+        raise MemoryError(f"RAM pressure – new browser denied: {e}")
+    except Exception as e:
+        raise RuntimeError(f"Failed to start browser: {e}")
+    finally:
+        if sig in POOL:
+            LAST_USED[sig] = time.time()
+        else:
+            # If we failed to start the browser, we should remove it from the pool
+            POOL.pop(sig, None)
+            LAST_USED.pop(sig, None)
+        # If we failed to start the browser, we should remove it from the pool
+async def close_all():
+    async with LOCK:
+        await asyncio.gather(*(c.close() for c in POOL.values()), return_exceptions=True)
+        POOL.clear(); LAST_USED.clear()
+
+async def janitor():
+    while True:
+        await asyncio.sleep(60)
+        now = time.time()
+        async with LOCK:
+            for sig, crawler in list(POOL.items()):
+                if now - LAST_USED[sig] > IDLE_TTL:
+                    with suppress(Exception): await crawler.close()
+                    POOL.pop(sig, None); LAST_USED.pop(sig, None)