Ask before a research run exceeds a cost limit

TL;DR

A single request can fan out to several paid DataForSEO calls and nothing shows the price before it runs. Show the estimated total first and, when it is above a limit the user set, wait for approval instead of spending.

What is happening?

Today any research request runs and bills without a pre-run estimate. A broad keyword request, an audit, or a few MCP calls from an agent can add up quickly, and self-hosters pay DataForSEO directly with no per-run control. There is no way to say "ask me first if a run costs more than $X".

What should happen?

Before a paid request executes, show its estimated cost. Under the limit, run as before. Over the limit, pause and ask: the user approves or cancels in the UI, and an agent receives a clear "approval needed" response with the estimate it can present, rather than spending silently. A declined request costs nothing.

Extra context

Cost visibility is tracked separately in #268, and #229 covers the hosted free tool's daily cap. A per-project limit with a sensible default would be enough for a first version. We have a working version of the approval check on a self-hosted deployment and can record a short demo if that helps.