fix(build): pin transformers in MLX requirements to prevent 5.x upgrade (#481)

mlx-audio depends on `transformers` with no upper bound. Installing
requirements-mlx.txt after requirements.txt lets pip upgrade transformers
past the 4.57.x cap to 5.x, which breaks three engines in the frozen MLX
bundle:

- qwen-custom-voice: `check_model_inputs` was rewritten to take `func` as
  positional, so `@check_model_inputs()` factory calls fail with
  `TypeError: missing 1 required positional argument: 'func'`
- tada-1b: `PretrainedConfig.__init_subclass__` now applies `@dataclass`,
  which rejects tada's `strides: list = []` mutable default
- luxtts: Whisper init hits `AssertionError` in `torch._refs.normal_`

Restating the same constraint here keeps mlx-audio's transformers
dependency from quietly winning the resolver.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jamiepine

backend/requirements-mlx.txt

@@ -3,3 +3,11 @@
 
 mlx>=0.30.0
 mlx-audio>=0.3.1
+
+# Restate the transformers cap from requirements.txt. mlx-audio depends on
+# `transformers` with no upper bound, so installing it after requirements.txt
+# lets pip upgrade transformers past 4.57.x (latest is 5.x), which breaks
+# qwen-custom-voice (`@check_model_inputs()` API change), tada-1b (dataclass
+# `mutable default` enforcement), and luxtts (Whisper init path) in the
+# frozen MLX bundle. Keep this constraint in sync with requirements.txt.
+transformers>=4.36.0,<=4.57.6