Add three-stage cross-domain news research and reading briefings

Group original news into tagged fact packages, review each package with evidence and read-only research, and build complete 72-hour reading lists with independent model selections.

Add paused RSS/Atom subscriptions, news-scoped Skills, versioned analysis records, and locked deployment support for SearXNG. Preserve original news and keep live migration and legacy AI cleanup separately authorized.

Validate with 604 backend tests, 49 frontend tests, 19 CI control tests, lint, type checks, production build, and disposable PostgreSQL migration and three-stage smoke checks.

backend/src/alphapulse/core/config.py

@@ -7,6 +7,7 @@ class Settings(BaseSettings):
     # 容器内由 compose 的 env_file 注入；只有一套配置，见仓库根目录 .env
     model_config = SettingsConfigDict(extra="ignore")
 
+    searxng_url: str = "http://searxng:8080"
     database_url: str
     redis_url: str
     jwt_secret: str