Streaming stdout for long-running commands (sync, webhook, daemon)

[longevityboris]

Commands that take time (`sync`, especially cold-start multi-account sync) emit nothing until done, then fire one JSON envelope. Clients with UI progress indicators have nothing to render.

Proposal

Add `--stream` flag that emits NDJSON events as they happen:

```jsonl
{"event":"sync.started","accounts":6}
{"event":"sync.account.done","account":"x@y.com","received":2,"sent":0}
{"event":"sync.account.done","account":"a@b.com","received":0,"sent":1}
...
{"event":"sync.completed","total_received":5,"total_sent":1}
```

• Non-streaming default stays as-is (one JSON envelope at exit).
• Streaming is opt-in, so existing scripts don't break.
• Minimail can render "Syncing 3/6 · alice@x.com" in real time.