feat(python): add AsyncInkbox awaitable client

[ron-42]

Fixes #146

The gap

The Python SDK is sync-only (httpx.Client). TypeScript is natively await. Async Python agents — FastAPI webhooks, ASGI tunnels, browser-use — wrap every call by hand:

await asyncio.to_thread(identity.send_email, ...)

Miss one site and the event loop blocks. In-repo examples already do this boilerplate (examples/use-inkbox-browser-use).

What this PR adds

AsyncInkbox / AsyncAgentIdentity — same constructor kwargs and public surface as sync Inkbox / AgentIdentity, but awaitable:

from inkbox import AsyncInkbox

async with AsyncInkbox(api_key="ApiKey_...") as inkbox:
    identity = await inkbox.create_identity("support-bot")
    await identity.send_email(
        to=["user@example.com"],
        subject="Hello",
        body_text="Hi",
    )

• Resource accessors (inkbox.messages, inkbox.vault, …) and nested objects return awaitable proxies.
• Sync iterators (iter_emails, …) are drained off the event loop and returned as lists.
• Escape hatches: inkbox.sync / identity.sync.
• Signup classmethods (signup, verify_signup, …) are awaitable on AsyncInkbox.

Implementation note. REST still uses the sync client via asyncio.to_thread, so the event loop does not block without cloning every resource onto httpx.AsyncClient. True async I/O is a follow-up if thread-pool load becomes a problem (called out in the module docstring). Sync Inkbox is unchanged.

Verification

pytest tests/test_async_client.py
# 7 passed

Live smoke against a real org (admin key): whoami, get_identity, send_email (to self), iter_unread_emails, get_message, mark_emails_read, list_identities, messages.list, vault.info — all via await.

[ron-42]

Hi, @alex-w-99 @dimavrem22. Kindly review the PR.