feat(api): add admin endpoint to issue keys for masters

Add `POST /admin/masters/{id}/keys` allowing admins to issue child keys
on behalf of a master. Introduce an `issued_by` field in the Key model
to audit whether a key was issued by the master or an admin.

Refactor master service to use typed errors for consistent HTTP status
mapping and ensure validation logic (active status, group check) is
shared.

cmd/server/main.go

@@ -158,6 +158,7 @@ func main() {
 	adminGroup.Use(middleware.AdminAuthMiddleware(adminService))
 	{
 		adminGroup.POST("/masters", adminHandler.CreateMaster)
+		adminGroup.POST("/masters/:id/keys", adminHandler.IssueChildKeyForMaster)
 		adminGroup.GET("/features", featureHandler.ListFeatures)
 		adminGroup.PUT("/features", featureHandler.UpdateFeatures)
 		// Other admin routes for managing providers, models, etc.