feat: Rust refactor — full backend rewrite (Phases 1-6)

Phase 1-3: Scaffold + types + auth (JWT, Argon2, API keys)
Phase 4a: Admin API (sites/accounts/tokens/downstream-keys/settings)
Phase 5: Proxy core (/v1/chat, /v1/images, /v1/models)
Phase 6: ComfyUI proxy routes migrated

- 80+ source files, all cargo check + 18 tests green
- Drop-in compatible with Node.js API surface

boosapi-rs/.env.example

@@ -0,0 +1,8 @@
+BOOSAPI_SERVER__HOST=0.0.0.0
+BOOSAPI_SERVER__PORT=4000
+BOOSAPI_SERVER__LOG_LEVEL=info
+BOOSAPI_DB__URL=sqlite://./data/boosapi.db
+BOOSAPI_DB__MAX_CONNECTIONS=10
+BOOSAPI_AUTH__JWT_SECRET=change-me
+BOOSAPI_AUTH__AUTH_TOKEN=admin
+BOOSAPI_AUTH__PROXY_TOKEN=proxy