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_proxy_list_download.py

@@ -13,7 +13,7 @@ class FpwProxyListDownloadPlugin(BaseHTTPPlugin):
 
     def __init__(self):
         super().__init__()
-        self.max_concurrency = 4
+        self.max_concurrency = 2
         self.api_pairs = [
             ("http", "https://www.proxy-list.download/api/v1/get?type=http"),
             ("https", "https://www.proxy-list.download/api/v1/get?type=https"),