/// Admin server: localhost-only config management, request logging, WebSocket live updates.
/// Backend HTTP clients for OpenAI, Vertex, Gemini, and Anthropic passthrough.
/// Async batch job submission and management (US3).
/// Response caching with in-memory (moka) and optional Redis tier (US1).
/// Webhook callback support for request completion notifications.
/// Environment-based configuration, TLS client cert setup, URL validation.
/// Per-request cost tracking and model pricing (US4).
/// Pure env-file parser (no I/O, no set_var). Used by startup bootstrap and admin import endpoint.
/// Backend fallback chains for transparent failover (US2).
/// Named integration registry (Langfuse, etc.).
/// Request count, success/error tracking, exposed via GET /metrics.
/// Optional OpenTelemetry OTLP trace export (requires `otel` feature).
/// Distributed rate limiting via Redis sorted sets (requires `redis` feature).
/// In-process chat completion runtime without HTTP route ownership.
/// Axum HTTP server: routes, middleware (auth, request ID, size/concurrency limits), SSE streaming.
/// Optional built-in server-side tools and registry.