feat(api): add alert system with CRUD endpoints and statistics

Introduce a comprehensive alert management system for monitoring
system events and notifications.

Changes include:
- Add Alert model with type, severity, status, and metadata fields
- Implement AlertHandler with full CRUD operations (create, list,
  get, acknowledge, resolve, dismiss)
- Add alert statistics endpoint for counts by status and severity
- Register Alert model in database auto-migration
- Add minute-level aggregation to log stats (limited to 6-hour range)

internal/model/alert.go

@@ -0,0 +1,56 @@
+package model
+
+import (
+	"time"
+
+	"gorm.io/gorm"
+)
+
+// AlertType defines the type of alert
+type AlertType string
+
+const (
+	AlertTypeRateLimit     AlertType = "rate_limit"
+	AlertTypeErrorSpike    AlertType = "error_spike"
+	AlertTypeQuotaExceeded AlertType = "quota_exceeded"
+	AlertTypeKeyDisabled   AlertType = "key_disabled"
+	AlertTypeKeyExpired    AlertType = "key_expired"
+	AlertTypeProviderDown  AlertType = "provider_down"
+)
+
+// AlertSeverity defines the severity level of an alert
+type AlertSeverity string
+
+const (
+	AlertSeverityInfo     AlertSeverity = "info"
+	AlertSeverityWarning  AlertSeverity = "warning"
+	AlertSeverityCritical AlertSeverity = "critical"
+)
+
+// AlertStatus defines the status of an alert
+type AlertStatus string
+
+const (
+	AlertStatusActive       AlertStatus = "active"
+	AlertStatusAcknowledged AlertStatus = "acknowledged"
+	AlertStatusResolved     AlertStatus = "resolved"
+	AlertStatusDismissed    AlertStatus = "dismissed"
+)
+
+// Alert represents a system alert or notification
+type Alert struct {
+	gorm.Model
+	Type        AlertType     `gorm:"size:50;not null;index" json:"type"`
+	Severity    AlertSeverity `gorm:"size:20;not null;index" json:"severity"`
+	Status      AlertStatus   `gorm:"size:20;not null;default:'active';index" json:"status"`
+	Title       string        `gorm:"size:255;not null" json:"title"`
+	Message     string        `gorm:"type:text" json:"message"`
+	RelatedID   uint          `gorm:"index" json:"related_id,omitempty"`
+	RelatedType string        `gorm:"size:50" json:"related_type,omitempty"` // master, key, apikey, provider_group
+	RelatedName string        `gorm:"size:255" json:"related_name,omitempty"`
+	Metadata    string        `gorm:"type:text" json:"metadata,omitempty"` // JSON encoded additional data
+	AckedAt     *time.Time    `json:"acked_at,omitempty"`
+	AckedBy     string        `gorm:"size:100" json:"acked_by,omitempty"`
+	ResolvedAt  *time.Time    `json:"resolved_at,omitempty"`
+	ExpiresAt   *time.Time    `gorm:"index" json:"expires_at,omitempty"` // Auto-dismiss after this time
+}