Add forward endpoint and forward-only processing for custom losses

[benji-cannot-code]

Summary

• add a /api/v1/forward endpoint and advertise it in the root API description
• extend the engine with a forward-only execution path and scheduler support so clients can fetch logprobs without accumulating gradients
• cover the new forward path with a unit test

Testing

• pytest tests/tinker/test_engine.py::test_forward_batch_produces_logprobs_without_accumulating_grads (fails: ModuleNotFoundError: No module named 'tx')

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Codex Task