fix(iroh-docs): Add flush_store and use it to make sure the default author is persisted

[matheus23]

Description

• Add a flush_store function to the docs actor/handle
• Use docs_store.flush_store().await? before persisting the default author ID file

There's two pieces of data related to the default author:

• The default author ID file
• The actual default author (the secret key) in redb

The code to store these two pieces of data looked like this:

docs_store.import_author(author).await?;
self.persist(author_id).await?;

So it would look like we make sure the author is stored before we store the file (which is basically just a reference to the author in the DB).

However, import_author is somewhat lazy, because it batches operations together in a bigger transaction.
The store would actually only get flushed in e.g. shutdown. So most of the time, the actual persisting of the author would happen after the call to self.persist.

If you wait for shutdown every time, it's all good, because that'll call flush. If not, then there will be some issues.

---

The fix in this PR is to introduce a flush_store function so we can use it after import_author to make sure it's actually flushed before we self.persist(author_id).

Breaking Changes

None. Only an addition: iroh-docs::actor::SyncHandle::flush_store.

Notes & open questions

There might still be something wrong with shutdown not running correctly, but IMO this is an improvement regardless.

Closes #2462

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• [ ] Tests if relevant.
• All breaking changes documented.

[matheus23]

Before this PR, the cli_rpc_lock_restart test would fail "reliably".
After this PR, it runs fine locally.
It may still be flaky. I don't know. But at least it's not "broken for sure".