fix: block private URLs in MCP tools

[RitwijParmar]

Summary

• add a URL guard that rejects localhost, private IPv4 ranges, link-local metadata IPs, loopback/link-local/unique-local IPv6, and IPv4-mapped private IPv6 targets by default
• apply the guard before forwarding scrape, map, crawl, and extract requests to Firecrawl
• allow trusted self-hosted/private-network deployments to opt in with FIRECRAWL_ALLOW_PRIVATE_NETWORKS=true

Why

This addresses the SSRF/local-network risk raised in #210 and #194. Shape validation with z.string().url() still allows targets like localhost, 127.0.0.1, 169.254.169.254, and private LAN ranges, so an MCP client can accidentally or maliciously route a tool call toward local infrastructure.

Verification

• npm run build
• node --test tests/urlGuard.test.mjs
• git diff --check

Note: I also tried the repo lint command, but a fresh install does not currently include the ESLint/@typescript-eslint dev dependencies required by .eslintrc.json, so lint could not be run locally without changing dependency metadata.