docs: refresh architecture.md to align with current codebase (#232)

- Update product positioning: AI Gateway (multi-deployment), not 'local-only proxy'
- Rewrite module tree: dispatcher/, planner/, ingress/<family>/, plugin/, ir/
- Update type names: EndpointHandler/Vendor/AiRequest/AiResponse/AiStreamDelta
- Add §4 request lifecycle & extension framework (five phases, PhaseHook, etc.)
- Update codec section: EndpointRegistration, per-family codec dirs, 6 codec traits
- Vendor layer: Vendor/VendorRegistry/dual-registration, VendorExtension, vertexai
- Routing renamed to models: models/model_backends/api_key_models, enable_auth/balance
- Storage schema rewrite: final table names, request_logs new columns, MySQL/oauth
- Future Work: mark pass-through done, OTel as foundation-ready, router partial

Author: shuaijinchao