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/pkg/tokenizer/tokenizer.go

@@ -0,0 +1,86 @@
+package tokenizer
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/pkoukk/tiktoken-go"
+)
+
+// Count 计算字符串的 token 数量
+func Count(text, model string) int {
+	tkm, err := tiktoken.EncodingForModel(model)
+	if err != nil {
+		tkm, _ = tiktoken.GetEncoding("cl100k_base")
+	}
+	return len(tkm.Encode(text, nil, nil))
+}
+
+// Cost 计算模型调用成本（USD，按每百万 token 定价）
+func Cost(model string, inputTokens, outputTokens int) float64 {
+	var inputPrice, outputPrice float64
+
+	switch {
+	case strings.Contains(model, "gpt-4o-mini"):
+		inputPrice = 0.15
+		outputPrice = 0.60
+	case strings.Contains(model, "gpt-4o"):
+		inputPrice = 2.50
+		outputPrice = 10.00
+	case strings.Contains(model, "gpt-4-turbo"):
+		inputPrice = 10.00
+		outputPrice = 30.00
+	case strings.Contains(model, "gpt-4"):
+		inputPrice = 30.00
+		outputPrice = 60.00
+	case strings.Contains(model, "gpt-3.5-turbo"):
+		inputPrice = 0.50
+		outputPrice = 1.50
+	case strings.Contains(model, "claude-3-5-sonnet"):
+		inputPrice = 3.00
+		outputPrice = 15.00
+	case strings.Contains(model, "claude-3-opus"):
+		inputPrice = 15.00
+		outputPrice = 75.00
+	case strings.Contains(model, "claude-3-haiku"):
+		inputPrice = 0.25
+		outputPrice = 1.25
+	case strings.Contains(model, "claude"):
+		inputPrice = 8.00
+		outputPrice = 24.00
+	case strings.Contains(model, "gemini-1.5-pro"):
+		inputPrice = 3.50
+		outputPrice = 10.50
+	case strings.Contains(model, "gemini-1.5-flash"):
+		inputPrice = 0.35
+		outputPrice = 0.53
+	case strings.Contains(model, "gemini"):
+		inputPrice = 0.50
+		outputPrice = 1.50
+	default:
+		inputPrice = 0.15
+		outputPrice = 0.60
+	}
+
+	cost := float64(inputTokens)/1e6*inputPrice + float64(outputTokens)/1e6*outputPrice
+	if cost < 0.000001 {
+		cost = 0.000001
+	}
+	return cost
+}
+
+// CostWithModel 从数据库模型记录获取定价
+func CostWithModel(inputTokens, outputTokens, cacheReadTokens, cacheCreationTokens int64, inputPrice, outputPrice, cacheReadPrice float64) float64 {
+	cost := float64(inputTokens)/1e6*inputPrice +
+		float64(outputTokens)/1e6*outputPrice +
+		float64(cacheReadTokens)/1e6*cacheReadPrice +
+		float64(cacheCreationTokens)/1e6*inputPrice*1.25
+	if cost < 0.000001 {
+		cost = 0.000001
+	}
+	return cost
+}
+
+func init() {
+	_ = fmt.Sprintf // ensure fmt is used
+}