feat: Enhance proxy clearance management with scheduler and warm-up functionality

[chenyme]

Summary

• Add a single-flight guard for proxy clearance refreshes so only one coroutine calls FlareSolverr per affinity key while concurrent requests wait for the same result.
• Add explicit clearance lifecycle helpers to invalidate and warm up cached bundles, and update the proxy clearance scheduler to prewarm on startup and refresh by invalidating then rebuilding bundles.
• Start the proxy clearance scheduler during app startup on the leader worker so FlareSolverr-managed clearance stays fresh in the background and the first real request is less likely to block on clearance generation.

Testing

• uv run python -c 'from app.control.proxy import ProxyDirectory; from app.control.proxy.scheduler import ProxyClearanceScheduler; d = ProxyDirectory(); s = ProxyClearanceScheduler(d); print("proxy scheduler import ok")'
• uv run python -c 'from app.main import create_app; app = create_app(); print(app.title)'

Related

• bug: Flaresolverr 似乎并不根据面板上设置的时间间隔自动触发刷新 #448