feat: add unified API gateway crate with REST endpoints
Implements Phase 2 REST API foundation:
- Unified gateway server with Axum web framework
- Complete REST endpoints for all services:
- Wallet (create, import, balance, sign, transactions)
- RPC (blocks, transactions, network, mempool)
- Storage (upload, download, pinning, CAR files)
- DEX (markets, orders, pools, liquidity)
- IBC (chains, channels, transfers, packets)
- ZK (circuits, proofs, ceremonies)
- Compiler (compile, ABI, analysis, validation)
- Authentication (JWT + API key)
- Rate limiting with tiered access
- CORS, security headers, request tracing
- Health check endpoints
- OpenAPI documentation scaffolding
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