fastapi: reload app after Gel config update (#854)

Use [the config hook](https://github.com/geldata/gel-cli/pull/1726) to
trigger a FastAPI app reload, so that the user could for example:

1. User updates `gel.local.toml` to modify the SMTP server port
2. `gel watch --sync` applies such config
3. Config hook runs to modify a signal file
4. `fastapi dev` is woken to reload the app