refactor: complete backend rewrite for multi-protocol proxy

Major rewrite of the Go backend to support:
- Three API format imports: openai, anthropic, compatible
- Three protocol conversions: Chat Completions, Responses, Messages
- Hub-and-spoke architecture with Chat as intermediate format

Deleted:
- opencat.go (old entry)
- store/, team/, pkg/team/, pkg/store/ (old data layer)
- internal/model/, internal/consts/ (old types)
- internal/service/team/, internal/controller/team/ (old handlers)
- llm/ (removed LLM client library, pure proxy mode)
- dist/, assets/ (old build artifacts)

Added:
- internal/store/ — 9 GORM models + multi-DB support
- internal/pkg/ — crypto (AES-GCM), apikey, jwt, ratelimit, resp, tokenizer
- internal/channel/ — channel selection, weighted LB, health checks
- internal/proxy/convert/ — 6 protocol conversion functions + SSE streaming
- internal/proxy/ — gateway with request dispatch and upstream selection
- internal/usage/ — async usage recorder with batch writes
- internal/api/ — management API (auth, users, keys, channels, models)
- Makefile for build/test/deploy

Fixed API to match frontend expectations:
- Login response wraps token in { data: { token } }
- GET /api/profile route added
- Profile response wraps user in { code, data }
- Role returned as number (10=admin, 1=user)

README.md

@@ -8,7 +8,7 @@
 
 [![Telegram group](https://img.shields.io/badge/dynamic/json?url=https%3A%2F%2Fapi.swo.moe%2Fstats%2Ftelegram%2FOpenTeamChat&query=count&color=2CA5E0&label=Telegram%20Group&logo=telegram&cacheSeconds=3600&style=flat-square)](https://t.me/OpenTeamChat) [![Telegram channel](https://img.shields.io/badge/dynamic/json?url=https%3A%2F%2Fapi.swo.moe%2Fstats%2Ftelegram%2FOpenTeamLLM&query=count&color=2CA5E0&label=Telegram%20Channel&logo=telegram&cacheSeconds=3600&style=flat-square)](https://t.me/OpenTeamLLM) 
 
-opencatd-open is an open-source, team-shared service for ChatGPT API that can be safely shared with others for API usage.
+opencatd-open is an open-source, team-shared service for OpenAI-compatible LLM APIs — route clients to any provider through a single endpoint, share costs, and track usage across your team.
 
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 OpenCat for Team的开源实现