refactor(crawl): parallel plugins via JobExecutor; per-plugin throttle

- Remove global crawl_slot gate; all CrawlJobs share only executor semaphore
- max_concurrent_jobs = max(24, n_plugins+8) for crawl-all + aggregator headroom
- BaseHTTPPlugin max_concurrency 3->2; fpw multi-URL plugins 4->2
- fetch_all: short random delay before each request to ease single-host pressure

Made-with: Cursor

app/plugins/fpw_socks_ssl_proxy.py

@@ -14,7 +14,7 @@ class FpwSocksSslProxyPlugin(BaseHTTPPlugin):
 
     def __init__(self):
         super().__init__()
-        self.max_concurrency = 4
+        self.max_concurrency = 2
         # 与 sslproxies 同模板的镜像站较多，socks-proxy 在部分网络下不稳定，多源提高成功率
         self.urls = [
             "https://www.sslproxies.org/",