feat(models): add Roman Hinglish Whisper model

[Mr-Sunglasses]

Problem

VocaPhone PR #218 adds an opt-in Roman Hinglish output mode backed by Oriserve/Whisper-Hindi2Hinglish-Apex. The gateway catalog does not currently expose that model, and passing its hinglish_roman output contract directly to whisper.cpp would use an invalid decoder language token.

Summary

• Add the pinned 574 MB ggml-apex-hinglish-q5_0.bin GGML model from the PR's immutable third-party conversion, with its SHA-256 pin and Apache-2.0 attribution.
• Advertise hinglish_roman as a Latin-script output contract and map it to Whisper's hi decoder token while preserving the contract in health/session responses.
• Accept the exact underscore-bearing output code without broadening language-tag validation to arbitrary underscore values.
• Reject incompatible explicit language requests and enforce Roman-only output even when a client requests auto, so Devanagari leakage cannot silently reach the cursor.
• Regenerate the model reference docs and keep the experimental model out of recommendation sets.

Verification

• just test — lint, WPS, mypy, package/audit, 508 tests, and Compose config passed
• uv run scripts/verify-model-pins.py --only whisper.cpp:ggml-apex-hinglish-q5_0.bin — 1 pin checked, 0 mismatched
• uv run scripts/generate_model_docs.py --check
• Container build — not applicable; no Dockerfile or lockfile changed

Privacy and security

• No secrets, recordings, transcripts, or private hostnames added
• No weakening of bearer auth, upload limits, retention, or default binding without discussion

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