fix(billing): 修复 OpenAI fast 档位计费并补齐展示

- 打通 service_tier 在 OpenAI HTTP、WS、passthrough 与 usage 记录中的传递
- 修正 priority/flex 计费逻辑，并将 fast 归一化为 priority
- 在用户端和管理端补齐服务档位与计费明细展示
- 补齐前后端测试，并修复 WS 限流信号重复持久化导致的全量回归失败

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/server/api_contract_test.go

@@ -210,8 +210,10 @@ func TestAPIContracts(t *testing.T) {
 							"sora_video_price_per_request": null,
 							"sora_video_price_per_request_hd": null,
 							"claude_code_only": false,
+						"allow_messages_dispatch": false,
 						"fallback_group_id": null,
 						"fallback_group_id_on_invalid_request": null,
+						"allow_messages_dispatch": false,
 						"created_at": "2025-01-02T03:04:05Z",
 						"updated_at": "2025-01-02T03:04:05Z"
 					}