feat(storage): add audit persistence

backend/internal/storage/migrations/001_create_audit_records.sql

@@ -0,0 +1,25 @@
+CREATE TABLE IF NOT EXISTS audit_records (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+
+    request_id TEXT NOT NULL UNIQUE,
+
+    created_at TEXT NOT NULL,
+
+    duration_ms INTEGER NOT NULL,
+
+    client_ip TEXT NULL,
+
+    final_status TEXT NOT NULL,
+
+    upstream_http_status INTEGER NULL,
+
+    masked_card TEXT NOT NULL,
+
+    response_json TEXT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_audit_records_created_at
+    ON audit_records(created_at);
+
+CREATE INDEX IF NOT EXISTS idx_audit_records_final_status
+    ON audit_records(final_status);