fix(backends): run MLX load and inference on one thread

MLX streams are thread-local and mlx-audio caches one on the model at
load time. Model load and inference were each dispatched through
asyncio.to_thread(), which uses a multi-worker pool, so load and
generate/transcribe could land on different OS threads -- the inference
thread then has no Stream(gpu, N) and MLX aborts with
"There is no Stream(gpu, 1) in current thread."

Route every MLX call (load, generate, transcribe; TTS and STT) through a
single dedicated worker thread so a model and its stream always share a
thread. max_workers=1 also serialises the single local GPU. Adds a
regression test covering the thread-affinity invariant.

Fixes #699. Also addresses #675.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

backend/tests/test_mlx_thread_affinity.py

@@ -0,0 +1,54 @@
+"""Regression tests for MLX worker-thread affinity (issues #675, #699)."""
+
+import asyncio
+import threading
+
+import pytest
+
+from backend.backends.mlx_backend import _run_on_mlx_thread
+
+
+class TestMLXThreadAffinity:
+    """MLX model load and inference must stay pinned to one OS thread.
+
+    MLX streams are thread-local and mlx-audio caches one at load time, so a
+    model loaded on one thread cannot be used from another -- inference aborts
+    with "There is no Stream(gpu, 1) in current thread". These tests lock in the
+    single-dedicated-thread guarantee that prevents that.
+    """
+
+    @pytest.mark.asyncio
+    async def test_all_calls_run_on_one_thread(self):
+        """Concurrently dispatched MLX calls all land on the same OS thread.
+
+        Dispatching concurrently (not sequentially) is what makes this prove a
+        single worker: with more than one worker the gather would fan out across
+        threads and the id set would have more than one entry.
+        """
+        thread_ids = await asyncio.gather(*[_run_on_mlx_thread(threading.get_ident) for _ in range(25)])
+
+        assert len(set(thread_ids)) == 1, f"MLX work spread across threads: {set(thread_ids)}"
+
+    @pytest.mark.asyncio
+    async def test_thread_local_stream_survives_across_calls(self):
+        """A stream cached at load time stays visible to later inference calls.
+
+        Mirrors mlx-audio: the stream is created on the load thread and reused
+        on every generate. On a different thread the second step would raise,
+        reproducing the user-facing error verbatim.
+        """
+        stream_registry = threading.local()
+
+        def _load_model():
+            stream_registry.stream = "Stream(gpu, 1)"
+
+        def _generate():
+            stream = getattr(stream_registry, "stream", None)
+            if stream is None:
+                raise RuntimeError("There is no Stream(gpu, 1) in current thread.")
+            return stream
+
+        await _run_on_mlx_thread(_load_model)
+        results = [await _run_on_mlx_thread(_generate) for _ in range(10)]
+
+        assert results == ["Stream(gpu, 1)"] * 10