fix(watchlists): use /iserver/watchlists endpoint (not /iserver/account/watchlists) (#14)

[SAY-5]

Fixes #14. get_watchlists proxied to GET /iserver/account/watchlists, which returns 404 on the IBKR Client Portal Gateway from build 10.44.1d onwards. The current endpoint is GET /iserver/watchlists (response shape: user lists under data.user_lists[]).

This PR aligns both the FastAPI route path and the upstream call. Other endpoints in this file already correctly use /iserver/account/watchlist[/...] (singular) for per-watchlist operations, which the IBKR docs continue to support unchanged.

Test plan

• AST parse on mcp_server/routers/watchlists.py is clean
• Endpoint and tag/summary/description preserved; only the path string changed
• Other endpoints in the same router are unchanged

[rcontesti]

@SAY-5 thank you for your collaboration. Allow some days to look into the matter.