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/api/handler.go

@@ -74,6 +74,7 @@ func (h *Handler) CreateProvider(c *gin.Context) {
 		Status:         status,
 		AutoBan:        autoBan,
 		BanReason:      req.BanReason,
+		Weight:         req.Weight,
 	}
 	if !req.BanUntil.IsZero() {
 		tu := req.BanUntil.UTC()
@@ -162,6 +163,9 @@ func (h *Handler) UpdateProvider(c *gin.Context) {
 	if req.Models != nil {
 		update["models"] = strings.Join(req.Models, ",")
 	}
+	if req.Weight > 0 {
+		update["weight"] = req.Weight
+	}
 	if strings.TrimSpace(req.Group) != "" {
 		update["group"] = groupx.Normalize(req.Group)
 	}