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)

internal/proxy/convert/stream.go

@@ -0,0 +1,171 @@
+package convert
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+)
+
+// SSEWriter writes Server-Sent Events
+type SSEWriter struct {
+	writer io.Writer
+	flusher http.Flusher
+}
+
+// NewSSEWriter creates a new SSE writer
+func NewSSEWriter(w http.ResponseWriter) *SSEWriter {
+	flusher, _ := w.(http.Flusher)
+	return &SSEWriter{
+		writer:  w,
+		flusher: flusher,
+	}
+}
+
+// WriteEvent writes a single SSE event
+func (w *SSEWriter) WriteEvent(event string, data interface{}) error {
+	var dataStr string
+	switch v := data.(type) {
+	case string:
+		dataStr = v
+	default:
+		b, err := json.Marshal(v)
+		if err != nil {
+			return err
+		}
+		dataStr = string(b)
+	}
+
+	_, err := fmt.Fprintf(w.writer, "event: %s\ndata: %s\n\n", event, dataStr)
+	if err != nil {
+		return err
+	}
+
+	if w.flusher != nil {
+		w.flusher.Flush()
+	}
+	return nil
+}
+
+// WriteChunk writes a streaming chunk in SSE format
+func (w *SSEWriter) WriteChunk(chunk interface{}) error {
+	b, err := json.Marshal(chunk)
+	if err != nil {
+		return err
+	}
+
+	_, err = fmt.Fprintf(w.writer, "data: %s\n\n", string(b))
+	if err != nil {
+		return err
+	}
+
+	if w.flusher != nil {
+		w.flusher.Flush()
+	}
+	return nil
+}
+
+// WriteDone writes the [DONE] marker
+func (w *SSEWriter) WriteDone() error {
+	_, err := fmt.Fprintf(w.writer, "data: [DONE]\n\n")
+	if err != nil {
+		return err
+	}
+
+	if w.flusher != nil {
+		w.flusher.Flush()
+	}
+	return nil
+}
+
+// SSEParser parses Server-Sent Events from a reader
+type SSEParser struct {
+	reader *bufio.Reader
+}
+
+// NewSSEParser creates a new SSE parser
+func NewSSEParser(r io.Reader) *SSEParser {
+	return &SSEParser{
+		reader: bufio.NewReader(r),
+	}
+}
+
+// SSEEvent represents a parsed SSE event
+type SSEEvent struct {
+	Event string
+	Data  string
+}
+
+// ReadEvent reads the next SSE event
+func (p *SSEParser) ReadEvent() (*SSEEvent, error) {
+	event := &SSEEvent{}
+
+	for {
+		line, err := p.reader.ReadString('\n')
+		if err != nil {
+			return nil, err
+		}
+
+		line = strings.TrimRight(line, "\r\n")
+
+		if line == "" {
+			// Empty line means end of event
+			if event.Data != "" || event.Event != "" {
+				return event, nil
+			}
+			continue
+		}
+
+		if strings.HasPrefix(line, "event:") {
+			event.Event = strings.TrimSpace(line[6:])
+		} else if strings.HasPrefix(line, "data:") {
+			data := strings.TrimSpace(line[5:])
+			if event.Data != "" {
+				event.Data += "\n" + data
+			} else {
+				event.Data = data
+			}
+		}
+		// Ignore comments (lines starting with :) and unknown fields
+	}
+}
+
+// ParseChatStreamChunk parses an OpenAI Chat Completions streaming chunk
+func ParseChatStreamChunk(data string) (*ChatCompletionStreamChunk, error) {
+	if data == "[DONE]" {
+		return nil, io.EOF
+	}
+
+	var chunk ChatCompletionStreamChunk
+	err := json.Unmarshal([]byte(data), &chunk)
+	if err != nil {
+		return nil, err
+	}
+	return &chunk, nil
+}
+
+// ParseMessagesStreamEvent parses an Anthropic Messages streaming event
+func ParseMessagesStreamEvent(data string) (*AnthropicStreamEvent, error) {
+	var event AnthropicStreamEvent
+	err := json.Unmarshal([]byte(data), &event)
+	if err != nil {
+		return nil, err
+	}
+	return &event, nil
+}
+
+// ParseResponsesStreamChunk parses an OpenAI Responses API streaming chunk
+func ParseResponsesStreamChunk(data string) (*ResponsesStreamEvent, error) {
+	if data == "[DONE]" {
+		return nil, io.EOF
+	}
+
+	var event ResponsesStreamEvent
+	err := json.Unmarshal([]byte(data), &event)
+	if err != nil {
+		return nil, err
+	}
+	return &event, nil
+}