feat(core): implement sync outbox mechanism and refactor provider validation

- Introduce `SyncOutboxService` and model to retry failed CP-to-Redis sync operations
- Update `SyncService` to handle sync failures by enqueuing tasks to the outbox
- Centralize provider group and API key validation logic into `ProviderGroupManager`
- Refactor API handlers to utilize the new manager and robust sync methods
- Add configuration options for sync outbox (interval, batch size, retries)

internal/model/sync_outbox.go

@@ -0,0 +1,18 @@
+package model
+
+import "time"
+
+// SyncOutbox stores failed CP -> Redis sync operations for retry.
+type SyncOutbox struct {
+	ID           uint       `gorm:"primaryKey" json:"id"`
+	ResourceType string     `gorm:"size:50;index" json:"resource_type"`
+	Action       string     `gorm:"size:50" json:"action"`
+	ResourceID   *uint      `gorm:"index" json:"resource_id,omitempty"`
+	Payload      string     `gorm:"type:text" json:"payload,omitempty"`
+	RetryCount   int        `gorm:"default:0" json:"retry_count"`
+	LastError    string     `gorm:"type:text" json:"last_error,omitempty"`
+	NextRetryAt  *time.Time `gorm:"index" json:"next_retry_at,omitempty"`
+	Status       string     `gorm:"size:20;default:'pending'" json:"status"`
+	CreatedAt    time.Time  `json:"created_at"`
+	UpdatedAt    time.Time  `json:"updated_at"`
+}