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_test.go

@@ -58,22 +58,31 @@ func TestModelRegistry_RefreshAndRollback(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)
 	}
-	if err := db.Create(&model.Provider{
-		Name:   "p1",
-		Type:   "openai",
-		Group:  "rg",
-		Models: "gpt-4o-mini",
-		Status: "active",
+	group := model.ProviderGroup{
+		Name:    "rg",
+		Type:    "openai",
+		BaseURL: "https://api.openai.com/v1",
+		Models:  "gpt-4o-mini",
+		Status:  "active",
+	}
+	if err := db.Create(&group).Error; err != nil {
+		t.Fatalf("create provider group: %v", err)
+	}
+	if err := db.Create(&model.APIKey{
+		GroupID: group.ID,
+		APIKey:  "k",
+		Status:  "active",
 	}).Error; err != nil {
-		t.Fatalf("create provider: %v", err)
+		t.Fatalf("create api key: %v", err)
 	}
 	if err := db.Create(&model.Binding{
 		Namespace:     "ns",
 		PublicModel:   "m",
-		RouteGroup:    "rg",
+		GroupID:       group.ID,
+		Weight:        1,
 		SelectorType:  "exact",
 		SelectorValue: "gpt-4o-mini",
 		Status:        "active",