feat(ai): 新增预算状态 + 成本估算 API 端点

Phase 3 Task 25:
- GET /ai/budget/status — 租户月度预算状态和告警等级
- GET /ai/cost/estimate — 按分析类型+模型估算单次成本

crates/erp-ai/src/module.rs

@@ -359,5 +359,13 @@ impl AiModule {
                 "/ai/quota/summary",
                 axum::routing::get(crate::handler::quota_summary),
             )
+            .route(
+                "/ai/budget/status",
+                axum::routing::get(crate::handler::budget_status),
+            )
+            .route(
+                "/ai/cost/estimate",
+                axum::routing::get(crate::handler::cost_estimate),
+            )
     }
 }