feat(trading): add trade history and daily FIFO retention cron

- trading_trades table (migration 0001) persists every buy/sell via optional onTrade callback
- /history [n] command shows caller's last N trades (default 10, max 50), HTML-escaped
- daily cron at 0 17 * * * trims to 1000/user + 10000/global via FIFO delete
- persistence failure logs but does not fail the trade reply

src/modules/trading/retention.js

@@ -0,0 +1,94 @@
+/**
+ * @file retention — daily cron handler that trims trading_trades to enforce row caps.
+ *
+ * Strategy (two-pass):
+ *   1. Per-user pass: for each distinct user_id, delete rows beyond PER_USER_CAP
+ *      (keeps the newest N rows per user).
+ *   2. Global FIFO pass: delete any rows beyond GLOBAL_CAP across all users
+ *      (keeps the newest N rows globally).
+ *
+ * Uses a hybrid SELECT-then-DELETE approach so it works with both the real
+ * Cloudflare D1 binding and the in-memory fake-d1 used in unit tests.
+ *
+ * @typedef {import("../../db/sql-store-interface.js").SqlStore} SqlStore
+ */
+
+const TABLE = "trading_trades";
+
+/** Default per-user row cap. Exported for testability. */
+export const PER_USER_CAP = 1000;
+
+/** Default global row cap across all users. Exported for testability. */
+export const GLOBAL_CAP = 10000;
+
+/**
+ * Build a dynamic `DELETE FROM <table> WHERE id IN (?, ?, ...)` query.
+ * Returns [query, ids] tuple — ids passed as spread binds.
+ *
+ * @param {number[]} ids
+ * @returns {[string, number[]]}
+ */
+function buildDeleteByIds(ids) {
+  const placeholders = ids.map(() => "?").join(", ");
+  return [`DELETE FROM ${TABLE} WHERE id IN (${placeholders})`, ids];
+}
+
+/**
+ * Daily cron handler — trims trading_trades to enforce per-user and global caps.
+ *
+ * @param {any} _event — Cloudflare ScheduledEvent (unused; present for handler contract).
+ * @param {{ sql: SqlStore | null }} ctx
+ * @param {{ perUserCap?: number, globalCap?: number }} [caps] — override caps (for tests).
+ * @returns {Promise<void>}
+ */
+export async function trimTradesHandler(_event, { sql }, caps = {}) {
+  if (sql === null) {
+    console.log("[trim-trades] no D1 binding — skipping");
+    return;
+  }
+
+  const perUserCap = caps.perUserCap ?? PER_USER_CAP;
+  const globalCap = caps.globalCap ?? GLOBAL_CAP;
+
+  let perUserDeleted = 0;
+
+  // ── Pass 1: per-user trim ──────────────────────────────────────────────────
+  const userRows = await sql.all(`SELECT DISTINCT user_id FROM ${TABLE}`);
+
+  for (const row of userRows) {
+    const userId = row.user_id;
+
+    // Fetch IDs of rows that exceed the per-user cap (oldest rows = beyond OFFSET perUserCap).
+    const excessRows = await sql.all(
+      `SELECT id FROM ${TABLE} WHERE user_id = ? ORDER BY ts DESC LIMIT -1 OFFSET ?`,
+      userId,
+      perUserCap,
+    );
+
+    if (excessRows.length === 0) continue;
+
+    const ids = excessRows.map((r) => r.id);
+    const [query, binds] = buildDeleteByIds(ids);
+    const result = await sql.run(query, ...binds);
+    perUserDeleted += result.changes ?? ids.length;
+  }
+
+  console.log(`[trim-trades] per-user pass: deleted ${perUserDeleted} rows`);
+
+  // ── Pass 2: global FIFO trim ───────────────────────────────────────────────
+  const globalExcess = await sql.all(
+    `SELECT id FROM ${TABLE} ORDER BY ts DESC LIMIT -1 OFFSET ?`,
+    globalCap,
+  );
+
+  let globalDeleted = 0;
+  if (globalExcess.length > 0) {
+    const ids = globalExcess.map((r) => r.id);
+    const [query, binds] = buildDeleteByIds(ids);
+    const result = await sql.run(query, ...binds);
+    globalDeleted = result.changes ?? ids.length;
+  }
+
+  console.log(`[trim-trades] global pass: deleted ${globalDeleted} rows`);
+  console.log(`[trim-trades] total deleted: ${perUserDeleted + globalDeleted} rows`);
+}