Support syncing by label

[ithinkihaveacat]

# only sync messages with the label "Transactional"
$ uv run main.py sync --label Transactional --data-dir ./data

[marcboeker]

Thanks for the PR. I love the idea to filter emails before synching, but I think it would be more flexible if we simply add a query argument to the commandline tool. Then you could say

uv run main.oy sync --query "label:Transactional" --data-dir ./data

But you are also able to only sync emails without attachments, or for a specific user etc. WDYT?

[ithinkihaveacat]

@marcboeker Yes that makes sense, especially if that use case is documented in the README. I'll update this PR.

[ithinkihaveacat]

Updated PR.

[marcboeker]

Maybe we can simplify this a bit. WDYT?

try:
    service = _create_service(credentials)
    labels = get_labels(service)
    base_query = query or ""

    if full_sync:
        # Full sync uses only the base query
        query_str = base_query
        logging.info(f"Performing full sync with query: {query_str}")
    else:
        # Build after/before filters into one compound query
        last = db.last_indexed()
        first = db.first_indexed()
        filters = []
        if last:
            filters.append(f"after:{int(last.timestamp())}")
        if first:
            filters.append(f"before:{int(first.timestamp())}")

        query_str = " ".join([base_query] + filters).strip()
        logging.info(f"Fetching messages with compound query: {query_str}")

    all_message_ids = get_message_ids_from_gmail(
        service, query_str, check_shutdown
    )
except Exception:
    logging.exception("Failed to sync messages")