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/api/lifespan.py

@@ -106,8 +106,10 @@ async def lifespan(app: FastAPI):
         )
         await stack.enter_async_context(worker_pool)
 
-        # Job 执行器
-        executor = JobExecutor(worker_pool=worker_pool, max_concurrent_jobs=10)
+        # Job 执行器：槽位需覆盖「全部爬取」时 N 个 CrawlJob + 聚合任务 + 全量验证等
+        _n_plugins = len(registry.list_plugins())
+        _max_jobs = max(24, _n_plugins + 8)
+        executor = JobExecutor(worker_pool=worker_pool, max_concurrent_jobs=_max_jobs)
         await stack.enter_async_context(executor)
 
         # 插件运行器