Add BlackVault — Open-source proxy gateway for AI API keys

## BlackVault

**GitHub:** https://github.com/venkat22022202/black-vault
**Live Demo:** https://black-vault-murex.vercel.app
**License:** MIT

### Description

Open-source API key management platform with a proxy gateway. Instead of sharing real API keys with AI agents, you generate proxy tokens (`bvt_...`). BlackVault injects the real key server-side and forwards to OpenAI, Anthropic, or Google AI with full SSE streaming support.

Kill a session = instant access revocation. No key rotation needed.

### Key Features
- AES-256-GCM encrypted vault with per-user derived keys (HKDF)
- Proxy gateway supporting OpenAI, Anthropic, Google AI with SSE streaming
- Instant kill switch — revoke per-session, per-key, or everything
- Per-session cost tracking with model-level accuracy
- Activity audit trail for every action
- Self-hostable, open source, free during beta

### Tech Stack
Next.js 16 · React 19 · TypeScript · tRPC 11 · Drizzle ORM · Neon Postgres · Upstash Redis · Clerk Auth · Tailwind v4