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/chat.go

@@ -0,0 +1,61 @@
+package convert
+
+// ChatCompletionRequest represents an OpenAI Chat Completions request
+type ChatCompletionRequest struct {
+	Model              string        `json:"model"`
+	Messages           []Message     `json:"messages"`
+	Temperature        *float64      `json:"temperature,omitempty"`
+	TopP               *float64      `json:"top_p,omitempty"`
+	N                  *int          `json:"n,omitempty"`
+	Stream             bool          `json:"stream,omitempty"`
+	Stop               interface{}   `json:"stop,omitempty"`
+	MaxTokens          *int          `json:"max_tokens,omitempty"`
+	PresencePenalty     *float64      `json:"presence_penalty,omitempty"`
+	FrequencyPenalty   *float64      `json:"frequency_penalty,omitempty"`
+	LogitBias          map[string]int `json:"logit_bias,omitempty"`
+	User               string        `json:"user,omitempty"`
+	Tools              []Tool        `json:"tools,omitempty"`
+	ToolChoice         interface{}   `json:"tool_choice,omitempty"`
+	ResponseFormat     interface{}   `json:"response_format,omitempty"`
+	Seed               *int          `json:"seed,omitempty"`
+}
+
+// ChatCompletionResponse represents an OpenAI Chat Completions response
+type ChatCompletionResponse struct {
+	ID                string   `json:"id"`
+	Object            string   `json:"object"`
+	Created           int64    `json:"created"`
+	Model             string   `json:"model"`
+	Choices           []Choice `json:"choices"`
+	Usage             *Usage   `json:"usage,omitempty"`
+	SystemFingerprint string   `json:"system_fingerprint,omitempty"`
+}
+
+type Choice struct {
+	Index        int      `json:"index"`
+	Message      Message  `json:"message"`
+	FinishReason string   `json:"finish_reason"`
+}
+
+// ChatCompletionStreamChunk represents a streaming chunk
+type ChatCompletionStreamChunk struct {
+	ID                string          `json:"id"`
+	Object            string          `json:"object"`
+	Created           int64           `json:"created"`
+	Model             string          `json:"model"`
+	Choices           []StreamChoice  `json:"choices"`
+	Usage             *Usage          `json:"usage,omitempty"`
+	SystemFingerprint string          `json:"system_fingerprint,omitempty"`
+}
+
+type StreamChoice struct {
+	Index        int           `json:"index"`
+	Delta        StreamDelta   `json:"delta"`
+	FinishReason *string       `json:"finish_reason"`
+}
+
+type StreamDelta struct {
+	Role    string     `json:"role,omitempty"`
+	Content string     `json:"content,omitempty"`
+	ToolCalls []ToolCall `json:"tool_calls,omitempty"`
+}