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/ratelimit/ratelimit.go

@@ -0,0 +1,122 @@
+package ratelimit
+
+import (
+	"sync"
+	"time"
+)
+
+// Limiter 内存限流器
+type Limiter struct {
+	mu sync.Mutex
+
+	// 每用户每秒请求数
+	userRPS map[uint64]*tokenBucket
+
+	// 密钥每日请求计数
+	keyDailyReq map[uint64]*dailyCounter
+
+	// 密钥每日 token 计数
+	keyDailyTokens map[uint64]*dailyCounter
+}
+
+type tokenBucket struct {
+	tokens   float64
+	maxTokens float64
+	refillRate float64
+	lastRefill time.Time
+}
+
+type dailyCounter struct {
+	date  string
+	count int64
+}
+
+func New() *Limiter {
+	return &Limiter{
+		userRPS:       make(map[uint64]*tokenBucket),
+		keyDailyReq:   make(map[uint64]*dailyCounter),
+		keyDailyTokens: make(map[uint64]*dailyCounter),
+	}
+}
+
+// AllowRequest 检查用户级每秒请求限制
+func (l *Limiter) AllowRequest(userID uint64, rps int) bool {
+	if rps <= 0 {
+		return true
+	}
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	bucket, ok := l.userRPS[userID]
+	if !ok {
+		bucket = &tokenBucket{
+			tokens:     float64(rps),
+			maxTokens:  float64(rps),
+			refillRate: float64(rps),
+			lastRefill: time.Now(),
+		}
+		l.userRPS[userID] = bucket
+	}
+
+	now := time.Now()
+	elapsed := now.Sub(bucket.lastRefill).Seconds()
+	bucket.tokens += elapsed * bucket.refillRate
+	if bucket.tokens > bucket.maxTokens {
+		bucket.tokens = bucket.maxTokens
+	}
+	bucket.lastRefill = now
+
+	if bucket.tokens < 1 {
+		return false
+	}
+	bucket.tokens--
+	return true
+}
+
+// AllowRequestDaily 检查密钥每日请求配额
+func (l *Limiter) AllowRequestDaily(keyID uint64, quota int) bool {
+	if quota <= 0 {
+		return true
+	}
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	today := time.Now().UTC().Format("2006-01-02")
+	counter, ok := l.keyDailyReq[keyID]
+	if !ok || counter.date != today {
+		l.keyDailyReq[keyID] = &dailyCounter{date: today, count: 1}
+		return true
+	}
+	if counter.count >= int64(quota) {
+		return false
+	}
+	counter.count++
+	return true
+}
+
+// TokensUsed 返回密钥今日 token 用量
+func (l *Limiter) TokensUsed(keyID uint64) int64 {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	today := time.Now().UTC().Format("2006-01-02")
+	counter, ok := l.keyDailyTokens[keyID]
+	if !ok || counter.date != today {
+		return 0
+	}
+	return counter.count
+}
+
+// AddTokens 累加密钥今日 token 用量
+func (l *Limiter) AddTokens(keyID uint64, tokens int64) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	today := time.Now().UTC().Format("2006-01-02")
+	counter, ok := l.keyDailyTokens[keyID]
+	if !ok || counter.date != today {
+		l.keyDailyTokens[keyID] = &dailyCounter{date: today, count: tokens}
+		return
+	}
+	counter.count += tokens
+}