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/messages.go

@@ -0,0 +1,37 @@
+package convert
+
+// MessagesRequest represents an Anthropic Messages API request
+type MessagesRequest struct {
+	Model         string        `json:"model"`
+	Messages      []Message     `json:"messages"`
+	MaxTokens     int           `json:"max_tokens"`
+	System        interface{}   `json:"system,omitempty"` // string or []ContentPart
+	Temperature   *float64      `json:"temperature,omitempty"`
+	TopP          *float64      `json:"top_p,omitempty"`
+	TopK          *int          `json:"top_k,omitempty"`
+	StopSequences []string      `json:"stop_sequences,omitempty"`
+	Stream        bool          `json:"stream,omitempty"`
+	Tools         []Tool        `json:"tools,omitempty"`
+	ToolChoice    interface{}   `json:"tool_choice,omitempty"`
+	Metadata      interface{}   `json:"metadata,omitempty"`
+}
+
+// MessagesResponse represents an Anthropic Messages API response
+type MessagesResponse struct {
+	ID            string         `json:"id"`
+	Type          string         `json:"type"`
+	Role          string         `json:"role"`
+	Content       []ContentBlock `json:"content"`
+	Model         string         `json:"model"`
+	StopReason    string         `json:"stop_reason"`
+	StopSequence  string         `json:"stop_sequence,omitempty"`
+	Usage         Usage          `json:"usage"`
+}
+
+// AnthropicStreamEvent represents an Anthropic streaming event
+type AnthropicStreamEvent struct {
+	Type  string      `json:"type"`
+	Index int         `json:"index,omitempty"`
+	Delta *Delta     `json:"delta,omitempty"`
+	Usage *Usage     `json:"usage,omitempty"`
+}