修复资产计费边界风险

资产生成预扣费改为 fail-closed，避免钱包异常时继续调用外部生成

新增钱包退款 outbox，退款失败时本地落盘并后台重放

拼图首图后台任务改用 SpacetimeDB claim 表实现跨实例互斥

计费 ledger id 统一绑定 request_id，并让前端重试复用 x-request-id

同步 SpacetimeDB bindings、后端架构文档和 Hermes 决策记录

server-rs/crates/api-server/src/match3d.rs

@@ -1,4 +1,4 @@
-use std::{
+use std::{
     collections::BTreeMap,
     convert::Infallible,
     future::Future,
@@ -65,10 +65,7 @@ use spacetime_client::{
 
 use crate::{
     api_response::json_success_body,
-    asset_billing::{
-        execute_billable_asset_operation_with_cost, map_asset_operation_wallet_error,
-        should_skip_asset_operation_billing_for_connectivity,
-    },
+    asset_billing::{execute_billable_asset_operation_with_cost, map_asset_operation_wallet_error},
     auth::{AuthenticatedAccessToken, RuntimePrincipal},
     config::AppConfig,
     generated_asset_sheets::apply_generated_asset_sheet_green_screen_alpha,
@@ -354,13 +351,6 @@ impl Match3DItemAssetsGenerationPlan {
             Self::Replace(plan) => plan.requested_item_names.len(),
         }
     }
-
-    fn billing_fingerprint_source(&self) -> String {
-        match self {
-            Self::Append(plan) => format!("append:{}", plan.requested_item_names.join("|")),
-            Self::Replace(plan) => format!("replace:{}", plan.requested_item_names.join("|")),
-        }
-    }
 }
 
 fn serialize_match3d_generated_item_assets(assets: &[Match3DGeneratedItemAsset]) -> Option<String> {