feat(api-key): add independent quota and expiration support

This feature allows API Keys to have their own quota limits and expiration
times, independent of the user's balance.

Backend:
- Add quota, quota_used, expires_at fields to api_key schema
- Implement IsExpired() and IsQuotaExhausted() checks in middleware
- Add ResetQuota and ClearExpiration API endpoints
- Integrate quota billing in gateway handlers (OpenAI, Anthropic, Gemini)
- Include quota/expiration fields in auth cache for performance
- Expiration check returns 403, quota exhausted returns 429

Frontend:
- Add quota and expiration inputs to key create/edit dialog
- Add quick-select buttons for expiration (+7, +30, +90 days)
- Add reset quota confirmation dialog
- Add expires_at column to keys list
- Add i18n translations for new features (en/zh)

Migration:
- Add 045_add_api_key_quota.sql for new columns

backend/internal/service/api_key.go

@@ -2,6 +2,14 @@ package service
 
 import "time"
 
+// API Key status constants
+const (
+	StatusAPIKeyActive         = "active"
+	StatusAPIKeyDisabled       = "disabled"
+	StatusAPIKeyQuotaExhausted = "quota_exhausted"
+	StatusAPIKeyExpired        = "expired"
+)
+
 type APIKey struct {
 	ID          int64
 	UserID      int64
@@ -15,8 +23,53 @@ type APIKey struct {
 	UpdatedAt   time.Time
 	User        *User
 	Group       *Group
+
+	// Quota fields
+	Quota     float64    // Quota limit in USD (0 = unlimited)
+	QuotaUsed float64    // Used quota amount
+	ExpiresAt *time.Time // Expiration time (nil = never expires)
 }
 
 func (k *APIKey) IsActive() bool {
 	return k.Status == StatusActive
 }
+
+// IsExpired checks if the API key has expired
+func (k *APIKey) IsExpired() bool {
+	if k.ExpiresAt == nil {
+		return false
+	}
+	return time.Now().After(*k.ExpiresAt)
+}
+
+// IsQuotaExhausted checks if the API key quota is exhausted
+func (k *APIKey) IsQuotaExhausted() bool {
+	if k.Quota <= 0 {
+		return false // unlimited
+	}
+	return k.QuotaUsed >= k.Quota
+}
+
+// GetQuotaRemaining returns remaining quota (-1 for unlimited)
+func (k *APIKey) GetQuotaRemaining() float64 {
+	if k.Quota <= 0 {
+		return -1 // unlimited
+	}
+	remaining := k.Quota - k.QuotaUsed
+	if remaining < 0 {
+		return 0
+	}
+	return remaining
+}
+
+// GetDaysUntilExpiry returns days until expiry (-1 for never expires)
+func (k *APIKey) GetDaysUntilExpiry() int {
+	if k.ExpiresAt == nil {
+		return -1 // never expires
+	}
+	duration := time.Until(*k.ExpiresAt)
+	if duration < 0 {
+		return 0
+	}
+	return int(duration.Hours() / 24)
+}