feat(billing): 引入 model_id 精确计费查找路径，传播模型 ID 至用量事件与 report context

- UsageEventData 新增 model_id / global_model_id 字段，write.rs seed 结构体同步补充
- report_context 新增 model_id / global_model_id / global_model_name，各 payload 构建时从 candidate 传入
- event_enrichment 优先按 model_id 精确查找计费上下文，回退为按名称多轮查找（保留 NoRule 结果降级逻辑）
- BillingReadRepository 新增 find_model_context_by_model_id，memory/sql 分别实现；SQL 查询重构支持按 provider_model_name 和 mappings 匹配并按优先级排序
- pricing.rs 修复：model_tiered_pricing 为空 tiers 时回退到 default_tiered_pricing
- request_metadata 允许字段列表补充 model_id / global_model_id / global_model_name
- admin usage 路由信息输出及脱敏字段列表同步新增三个 model 相关字段

crates/aether-billing/src/pricing.rs

@@ -24,6 +24,7 @@ impl BillingModelPricingSnapshot {
     pub fn effective_tiered_pricing(&self) -> Option<&Value> {
         self.model_tiered_pricing
             .as_ref()
+            .filter(|value| has_tiered_pricing_tiers(value))
             .or(self.default_tiered_pricing.as_ref())
     }
 
@@ -58,6 +59,67 @@ impl BillingModelPricingSnapshot {
     }
 }
 
+fn has_tiered_pricing_tiers(value: &Value) -> bool {
+    value
+        .get("tiers")
+        .and_then(Value::as_array)
+        .is_some_and(|tiers| !tiers.is_empty())
+}
+
+#[cfg(test)]
+mod tests {
+    use serde_json::json;
+
+    use super::BillingModelPricingSnapshot;
+
+    fn snapshot(
+        model_tiered_pricing: Option<serde_json::Value>,
+        default_tiered_pricing: Option<serde_json::Value>,
+    ) -> BillingModelPricingSnapshot {
+        BillingModelPricingSnapshot {
+            provider_id: "provider-1".to_string(),
+            provider_billing_type: None,
+            provider_api_key_id: None,
+            provider_api_key_rate_multipliers: None,
+            provider_api_key_cache_ttl_minutes: None,
+            global_model_id: "global-model-1".to_string(),
+            global_model_name: "gpt-5".to_string(),
+            global_model_config: None,
+            default_price_per_request: None,
+            default_tiered_pricing,
+            model_id: Some("model-1".to_string()),
+            model_provider_model_name: Some("gpt-5-upstream".to_string()),
+            model_config: None,
+            model_price_per_request: None,
+            model_tiered_pricing,
+        }
+    }
+
+    #[test]
+    fn empty_provider_tiered_pricing_inherits_global_default() {
+        let default_pricing =
+            json!({"tiers":[{"up_to":null,"input_price_per_1m":3.0,"output_price_per_1m":15.0}]});
+        let pricing = snapshot(Some(json!({})), Some(default_pricing.clone()));
+
+        assert_eq!(pricing.effective_tiered_pricing(), Some(&default_pricing));
+
+        let pricing = snapshot(Some(json!({"tiers": []})), Some(default_pricing.clone()));
+
+        assert_eq!(pricing.effective_tiered_pricing(), Some(&default_pricing));
+    }
+
+    #[test]
+    fn populated_provider_tiered_pricing_overrides_global_default() {
+        let provider_pricing =
+            json!({"tiers":[{"up_to":null,"input_price_per_1m":1.0,"output_price_per_1m":2.0}]});
+        let default_pricing =
+            json!({"tiers":[{"up_to":null,"input_price_per_1m":3.0,"output_price_per_1m":15.0}]});
+        let pricing = snapshot(Some(provider_pricing.clone()), Some(default_pricing));
+
+        assert_eq!(pricing.effective_tiered_pricing(), Some(&provider_pricing));
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct BillingUsageInput {
     pub task_type: String,