refactor(api): split Provider into ProviderGroup and APIKey models

Restructure the provider management system by separating the monolithic
Provider model into two distinct entities:

- ProviderGroup: defines shared upstream configuration (type, base_url,
  google settings, models, status)
- APIKey: represents individual credentials within a group (api_key,
  weight, status, auto_ban, ban settings)

This change also updates:
- Binding model to reference GroupID instead of RouteGroup string
- All CRUD handlers for the new provider-group and api-key endpoints
- Sync service to rebuild provider snapshots from joined tables
- Model registry to aggregate capabilities across group/key pairs
- Access handler to validate namespace existence and subset constraints
- Migration importer to handle the new schema structure
- All related tests to use the new model relationships

BREAKING CHANGE: Provider API endpoints replaced with /provider-groups
and /api-keys endpoints; Binding.RouteGroup replaced with Binding.GroupID

internal/service/model_registry_check_test.go

@@ -23,7 +23,7 @@ func TestModelRegistry_Check(t *testing.T) {
 	if err != nil {
 		t.Fatalf("open sqlite: %v", err)
 	}
-	if err := db.AutoMigrate(&model.Provider{}, &model.Binding{}, &model.Model{}); err != nil {
+	if err := db.AutoMigrate(&model.ProviderGroup{}, &model.APIKey{}, &model.Binding{}, &model.Model{}); err != nil {
 		t.Fatalf("migrate: %v", err)
 	}