feat: decouple billing correctness from usage log batching

backend/migrations/071_add_usage_billing_dedup.sql

@@ -0,0 +1,13 @@
+-- 窄表账务幂等键：将“是否已扣费”从 usage_logs 解耦出来
+-- 幂等执行：可重复运行
+
+CREATE TABLE IF NOT EXISTS usage_billing_dedup (
+    id BIGSERIAL PRIMARY KEY,
+    request_id VARCHAR(255) NOT NULL,
+    api_key_id BIGINT NOT NULL,
+    request_fingerprint VARCHAR(64) NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_usage_billing_dedup_request_api_key
+    ON usage_billing_dedup (request_id, api_key_id);