fix: add Metal JIT warmup, cache inspect.signature, and add metal health label

[neuron-tech-ai]

Three small performance fixes for Metal (Apple Silicon) TTS:

1. Metal JIT warmup at startup — Metal compiles shaders JIT on first use, causing a multi-second stall on the first generation. Warming up at startup moves this cost to load time where it's expected, not to the first generation where it looks like a hang.

2. Cache inspect.signature() — this was being called on every single TTS generation to introspect the model's forward() signature. Caching the result eliminates the repeated reflection overhead.

3. Add metal health label — the /health endpoint reported backend type but didn't distinguish metal from other backends, making it harder to confirm Metal acceleration is active.

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Configuration used: defaults

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Run ID: c91205d4-359d-4608-8332-2403c620d3d6

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Reviewing files that changed from the base of the PR and between b35b909 and 7984cd4.

📒 Files selected for processing (2)

• backend/backends/mlx_backend.py
• backend/routes/health.py

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