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/dto/provider.go

@@ -13,6 +13,7 @@ type ProviderDTO struct {
 	Group          string    `json:"group"`
 	Models         []string  `json:"models"` // List of supported model names
 	Status         string    `json:"status"`
+	Weight         int       `json:"weight,omitempty"`
 	AutoBan        *bool     `json:"auto_ban,omitempty"`
 	BanReason      string    `json:"ban_reason,omitempty"`
 	BanUntil       time.Time `json:"ban_until,omitempty"`