feat(redirects): add composable proxy policies

[Cedarflake]

Summary

• Add composable proxy policy profiles while preserving the existing redirect JSON shape
• Separate proxy matching, request construction, header handling, and safety enforcement in Runtime
• Add schema validation, WebUI editing controls, bilingual documentation, and regression coverage

Validation

• Runtime type checking and full Runtime test suite
• Cloudflare, Vercel, and Netlify Runtime builds
• WebUI test suite, lint, message validation, and production build
• Config, plugin contract, and repository boundary checks
• git diff --check

pnpm data:validate could not resolve the local origin/data ref in this checkout; no remote data mutation was performed.