[feature] AI API Gateway

## Summary

Introduce an **AI API Gateway** as a first-class automation capability: a unified entry point for routing, governing, and observing LLM traffic across multiple providers and models.

## Capabilities

**Configuration**
- Providers (OpenAI, Anthropic, etc.) — CRUD
- Models registry — CRUD
- Routing policies — CRUD
- Budget configuration with enforcement modes
- Gateway settings

**Observability**
- Monitoring dashboard (requests, latency, errors, cost) via Recharts
- Workspace-scoped GraphQL operations for all gateway entities

**Integration**
- New sidebar entry in Automation (EE-gated)
- Liquibase schema migrations for gateway persistence

## Acceptance Criteria

- [ ] Design spec merged and reviewed
- [ ] Backend module boots with migrations applied cleanly
- [ ] All CRUD pages (Providers, Models, Routing Policies, Budget, Settings) functional
- [ ] Monitoring dashboard renders live metrics
- [ ] EE edition gating verified (hidden in CE)
- [ ] Integration tests for gateway services