feat(transcriber): add AsyncTranscriber for asyncio callers

[he-james]

Add aai.AsyncTranscriber, the asyncio version of aai.Transcriber. Every method that calls the API is a coroutine, so many transcriptions run concurrently on one thread. Addresses #80.

Transcriber.transcribe_async returns a concurrent.futures.Future. That future is not awaitable, and .result() blocks the event loop. An asyncio caller must therefore use a thread pool, and the hardware limits the thread count.

Updates

File	Change
assemblyai/async_client.py	New. AsyncClient wraps an httpx.AsyncClient.
assemblyai/async_api.py	New. Asyncio version of each transcript request function.
assemblyai/async_transcriber.py	New. AsyncTranscriber and AsyncTranscript.
assemblyai/types.py	1 line. LemurSource now accepts an AsyncTranscript.
assemblyai/__init__.py	Exports AsyncTranscriber, AsyncTranscript, AsyncClient.
README.md, CLAUDE.md	Asyncio examples and reference notes.
tests/unit/test_async_transcriber.py	New. 38 tests.

New functionality

• AsyncTranscriber: transcribe, submit, transcribe_group, submit_group, get_by_id, delete_by_id, list_transcripts, upload_file, aclose, and the async context manager protocol.
• AsyncTranscript: the same fields as Transcript. Only the methods that call the API are coroutines.
• AsyncClient: an httpx.AsyncClient pool. Share one pool between several transcribers.

import asyncio
import assemblyai as aai

async def main():
    async with aai.AsyncTranscriber() as transcriber:
        transcript = await transcriber.transcribe("./audio.mp3")
        print(transcript.text)
        sentences = await transcript.get_sentences()

asyncio.run(main())