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)

Makefile

@@ -0,0 +1,61 @@
+.PHONY: build run test clean fmt lint
+
+BINARY_NAME=openteam
+BUILD_DIR=bin
+
+# Build
+build:
+	go build -o $(BUILD_DIR)/$(BINARY_NAME) ./cmd/openteam
+
+# Run
+run: build
+	./$(BUILD_DIR)/$(BINARY_NAME)
+
+# Development run
+dev:
+	go run ./cmd/openteam
+
+# Test
+test:
+	go test ./internal/... -v
+
+# Test with coverage
+test-cover:
+	go test ./internal/... -coverprofile=coverage.out
+	go tool cover -html=coverage.out -o coverage.html
+
+# Format code
+fmt:
+	go fmt ./...
+
+# Lint
+lint:
+	golangci-lint run
+
+# Clean
+clean:
+	rm -rf $(BUILD_DIR)
+	rm -f coverage.out coverage.html
+
+# Tidy dependencies
+tidy:
+	go mod tidy
+
+# Build for Linux
+build-linux:
+	CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -o $(BUILD_DIR)/$(BINARY_NAME)-linux-amd64 ./cmd/openteam
+
+# Build for macOS
+build-mac:
+	CGO_ENABLED=0 GOOS=darwin GOARCH=arm64 go build -o $(BUILD_DIR)/$(BINARY_NAME)-darwin-arm64 ./cmd/openteam
+
+# Build all platforms
+build-all: build-linux build-mac
+
+# Database migration (will be implemented)
+migrate:
+	@echo "Migration will be implemented in future"
+
+# Seed data (will be implemented)
+seed:
+	@echo "Seeding will be implemented in future"