feat(cleanup): hide and prune stale mailboxes

Mailbox listings grew unbounded as old sessions ended without
unregistering. This adds two layers of cleanup, configurable via
mailbox.json or `serve` flags:

- Lazy filter: list responses (REST /v1/list, MCP list_mailboxes)
  drop mailboxes idle longer than hideAfterMinutes (default 24h),
  while always keeping the caller and any sender with messages
  pending for them.
- Background sweep: startServer runs an initial prune on boot and
  schedules an unref'd interval timer that hard-deletes mailboxes
  idle longer than deleteAfterMinutes (default 7d) which have no
  pending messages, and wipes their delivered history.

node/src/cli.ts

@@ -78,7 +78,30 @@ program
   .option("--bind <address>", "Bind address")
   .option("--db-path <path>", "SQLite database path")
   .option("--config <path>", "Path to mailbox.json")
-  .action(async (opts: { port?: number; bind?: string; dbPath?: string; config?: string }) => {
+  .option(
+    "--hide-after-minutes <n>",
+    "Hide mailboxes idle longer than N minutes from list responses (0 = disabled)",
+    (v) => parseInt(v, 10),
+  )
+  .option(
+    "--delete-after-minutes <n>",
+    "Hard-delete mailboxes idle longer than N minutes (0 = disabled)",
+    (v) => parseInt(v, 10),
+  )
+  .option(
+    "--sweep-interval-minutes <n>",
+    "Stale-mailbox sweep interval in minutes (0 = disabled)",
+    (v) => parseInt(v, 10),
+  )
+  .action(async (opts: {
+    port?: number;
+    bind?: string;
+    dbPath?: string;
+    config?: string;
+    hideAfterMinutes?: number;
+    deleteAfterMinutes?: number;
+    sweepIntervalMinutes?: number;
+  }) => {
     const cfg = resolveConfig(opts);
     try {
       const { startServer } = await import("./server.js");