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

tests/memory/cap_test.py

@@ -0,0 +1,34 @@
+#!/usr/bin/env python3
+"""
+Hammer /crawl with many concurrent requests to prove GLOBAL_SEM works.
+"""
+
+import asyncio, httpx, json, uuid, argparse
+
+API = "http://localhost:8020/crawl"
+URLS_PER_CALL = 1          # keep it minimal so each arun() == 1 page
+CONCURRENT_CALLS = 20      # way above your cap
+
+payload_template = {
+    "browser_config": {"type": "BrowserConfig", "params": {"headless": True}},
+    "crawler_config": {
+        "type": "CrawlerRunConfig",
+        "params": {"cache_mode": "BYPASS", "verbose": False},
+    }
+}
+
+async def one_call(client):
+    payload = payload_template.copy()
+    payload["urls"] = [f"https://httpbin.org/anything/{uuid.uuid4()}"]
+    r = await client.post(API, json=payload)
+    r.raise_for_status()
+    return r.json()["server_peak_memory_mb"]
+
+async def main():
+    async with httpx.AsyncClient(timeout=60) as client:
+        tasks = [asyncio.create_task(one_call(client)) for _ in range(CONCURRENT_CALLS)]
+        mem_usages = await asyncio.gather(*tasks)
+        print("Calls finished OK, server peaks reported:", mem_usages)
+
+if __name__ == "__main__":
+    asyncio.run(main())