refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/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"`
+}