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/responses.go

@@ -0,0 +1,59 @@
+package convert
+
+// ResponsesRequest represents an OpenAI Responses API request
+type ResponsesRequest struct {
+	Model           string      `json:"model"`
+	Input           []InputItem `json:"input"`
+	Instructions    string      `json:"instructions,omitempty"`
+	MaxOutputTokens *int        `json:"max_output_tokens,omitempty"`
+	Tools           []Tool      `json:"tools,omitempty"`
+	ToolChoice      interface{} `json:"tool_choice,omitempty"`
+	Stream          bool        `json:"stream,omitempty"`
+	Temperature     *float64    `json:"temperature,omitempty"`
+	TopP            *float64    `json:"top_p,omitempty"`
+	Metadata        interface{} `json:"metadata,omitempty"`
+}
+
+// InputItem represents a single input item
+type InputItem struct {
+	Role    string      `json:"role"`
+	Content interface{} `json:"content,omitempty"`
+}
+
+// ResponsesResponse represents an OpenAI Responses API response
+type ResponsesResponse struct {
+	ID        string        `json:"id"`
+	Object    string        `json:"object"`
+	CreatedAt int64         `json:"created_at"`
+	Status    string        `json:"status"`
+	Model     string        `json:"model"`
+	Output    []OutputItem  `json:"output"`
+	Usage     Usage         `json:"usage"`
+	Error     interface{}   `json:"error,omitempty"`
+	Incomplete *Incomplete  `json:"incomplete,omitempty"`
+}
+
+type OutputItem struct {
+	Type    string         `json:"type"`
+	Content []OutputContent `json:"content,omitempty"`
+	Role    string         `json:"role,omitempty"`
+}
+
+type OutputContent struct {
+	Type  string  `json:"type"`
+	Text  string  `json:"text,omitempty"`
+	ID    string  `json:"id,omitempty"`
+	Name  string  `json:"name,omitempty"`
+	Input interface{} `json:"input,omitempty"`
+}
+
+type Incomplete struct {
+	Reason string `json:"reason"`
+}
+
+// ResponsesStreamEvent represents a Responses API streaming event
+type ResponsesStreamEvent struct {
+	Type  string `json:"type"`
+	Item  *OutputItem `json:"item,omitempty"`
+	Delta string `json:"delta,omitempty"`
+}