feat: decouple billing correctness from usage log batching

backend/migrations/073_add_usage_billing_dedup_archive.sql

@@ -0,0 +1,10 @@
+-- 冷归档旧账务幂等键，缩小热表索引与清理范围，同时不丢失长期去重能力。
+
+CREATE TABLE IF NOT EXISTS usage_billing_dedup_archive (
+    request_id VARCHAR(255) NOT NULL,
+    api_key_id BIGINT NOT NULL,
+    request_fingerprint VARCHAR(64) NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL,
+    archived_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    PRIMARY KEY (request_id, api_key_id)
+);