feat(api): add provider creation endpoints and weight support

- Add `POST /admin/providers/preset` for streamlined creation of official providers (OpenAI, Anthropic, Gemini)
- Add `POST /admin/providers/custom` for generic OpenAI-compatible providers
- Add `weight` field to provider model and DTOs to enable weighted routing
- Update sync service to propagate provider weights
- Add unit tests for new creation handlers

internal/service/sync.go

@@ -79,6 +79,7 @@ func (s *SyncService) SyncProvider(provider *model.Provider) error {
 		GoogleLocation: provider.GoogleLocation,
 		Group:          group,
 		Models:         models,
+		Weight:         provider.Weight,
 		Status:         normalizeStatus(provider.Status),
 		AutoBan:        provider.AutoBan,
 		BanReason:      provider.BanReason,
@@ -139,6 +140,7 @@ type providerSnapshot struct {
 	GoogleLocation string   `json:"google_location,omitempty"`
 	Group          string   `json:"group"`
 	Models         []string `json:"models"`
+	Weight         int      `json:"weight,omitempty"`
 	Status         string   `json:"status"`
 	AutoBan        bool     `json:"auto_ban"`
 	BanReason      string   `json:"ban_reason,omitempty"`
@@ -220,6 +222,7 @@ func (s *SyncService) SyncAll(db *gorm.DB) error {
 			GoogleLocation: p.GoogleLocation,
 			Group:          group,
 			Models:         models,
+			Weight:         p.Weight,
 			Status:         normalizeStatus(p.Status),
 			AutoBan:        p.AutoBan,
 			BanReason:      p.BanReason,