Slack Install Auth Url

JSON variant of /install/start, mirroring routes/mcp_integrations.py's POST /{provider_key}/auth/url pattern.

GET /install/start is an authenticated redirect -- Depends(get_current_user_and_org) reads ONLY the Authorization header (no cookie or query-token fallback anywhere in auth.py), so a plain top-level browser navigation to it cannot carry that header and would 401 before ever reaching Slack. This route lets the frontend attach the bearer token via a normal authenticated fetch, then navigate with window.location.href to the returned THIRD-PARTY url -- which needs no bearer token, same as the MCP OAuth connect flow. /install/start is left working unchanged for any caller that can supply the header directly (e.g. a signed script or a future first-party client).