Add Cloudflare Pages deploy target

[dannywillems]

Summary

• Add CloudflarePages deployer for static site hosting via wrangler pages deploy
• Extend DnsProvider trait with CNAME record support (upsert/delete) with default error implementations so existing providers (OVH) don't break
• Implement CNAME methods for Cloudflare DNS provider
• Extend Deployer trait with is_remote() and cname_target() for local deployers
• Make host CLI argument optional for non-remote deployers
• Pipeline auto-creates CNAME records pointing to {project}.pages.dev after deploy

Usage

let web = App::new("my-site")
    .build_cmd("npm run build")
    .build_dir("dist");

let caddy = Caddy::new();

let pipeline = Pipeline::new(web, caddy)
    .dns(Cloudflare::new("my-site.example.com"))
    .deploy(CloudflarePages::new("my-project"));

pipeline.run()?;

Test plan

• cargo check compiles
• cargo clippy -- -D warnings passes
• cargo fmt --check passes
• cargo test all 107 tests pass (including doc-tests)
• cargo doc --no-deps builds successfully
• Manual test with a real Cloudflare Pages project and wrangler CLI

Closes #17