feat(store): add MaintenancePolicy CRUD backend

Типы maintenance_policy, методы store.Backend и реализации для PostgreSQL и in-memory.

Co-authored-by: Cursor <[email protected]>

internal/store/backend.go

@@ -115,6 +115,16 @@ type Backend interface {
 
 	// RunPeriodicMaintenance prunes stale DB rows (no-op for in-memory).
 	RunPeriodicMaintenance(ctx context.Context)
+
+	// Maintenance policies (instance-scoped PostgreSQL maintenance configuration).
+	ListMaintenancePolicies(cursor string, limit int) ([]*MaintenancePolicy, string, bool, error)
+	GetMaintenancePolicy(id string) (*MaintenancePolicy, error)
+	CreateMaintenancePolicy(in *MaintenancePolicy) (*MaintenancePolicy, error)
+	UpdateMaintenancePolicy(id string, patch *MaintenancePolicyPatch) (*MaintenancePolicy, error)
+	DeleteMaintenancePolicy(id string) error
+	TouchMaintenancePolicyRun(id, status, errMsg string) error
+	AppendMaintenancePolicyConfigAudit(actor, policyID, action string, before, after map[string]any) error
+	ListMaintenancePolicyConfigAudit(cursor string, limit int) ([]*MaintenancePolicyConfigAudit, string, bool, error)
 }
 
 // ASNPrefixCacheEntry is a cached RIPEstat response for one ASN.