fix: use utf-8 for local tracking text files

[Ghraven]

Summary

Thanks for maintaining Burr. This PR makes the local tracking backend text-file reads and writes use explicit UTF-8 encoding.

What changed

• Adds encoding="utf-8" to annotation JSONL reads and writes in LocalBackend
• Adds encoding="utf-8" when reading children.jsonl
• Leaves binary log/metadata reads unchanged

Before / after

Before, these JSONL text files used the platform default encoding, which can vary on Windows and other non-UTF-8 locale environments.

After, the local backend reads and writes these text files consistently as UTF-8.

Verification

• python -m py_compile burr/tracking/server/backend.py
• git diff --check

[andreahlert]

Thanks for the fix. One spot in the same file looks missed: backend.py:524 reads graph.json text-mode without encoding="utf-8". Same class of bug, and client writes it as UTF-8 (client.py:420).

async with aiofiles.open(graph_file, encoding="utf-8") as f:

Could you include that line in this PR as well?