feat: add backend provider model controls

[aditya-damerla128]

Summary

Adds the backend foundation for provider-aware model controls:

• provider/model registry rows for discovered or seeded models
• parent-key model restriction storage and manager admin endpoints
• effective model list endpoint for parent or agent keys
• chat request enforcement for { provider, model }
• per-request gateway provider routing with x-portkey-provider
• agent YAML parsing for provider, model, and allowedModels

This is intentionally a backend tracking PR. It does not include the manager UI or widget selector yet.

Refs #182

Validation

• npm test -- manager-auth.test.js
• npm run build
• npm run lint passes with existing warnings only