chore[notask]: revert Opus registry removal (premature — SDK still references them)

[Alok-Ranjan23]

Summary

Reverts commit 436e29c (PR #1602) which removed 32 Opus model entries from the registry server.

Why

The registry entries were removed before the SDK stopped referencing them. The published SDK (@qvac/sdk@0.8.3) still exports MARIAN_OPUS_* constants. Users on that version get broken model resolution because the registry no longer has the Opus entries.

Correct order:

1. First: SDK removes Opus constants (PR QVAC-14576 chore[bc]: remove Opus NMT engine from SDK API surface #1622)
2. Publish new SDK version
3. Then: remove Opus from registry

What's restored

• 32 Opus/Marian entries in data/models.prod.json (680 → 712)
• Marian attribution in NOTICE (Apache-2.0 + CC-BY-4.0 sections)
• 22 ggml-opus-* entries in client/NOTICE

[github-actions]

Tier-based Approval Status

**PR Tier:** TIER1

**Current Status:** ✅ APPROVED

**Requirements:**
- 1 Team Member approval ❌ (0/1)
- 1 Team Lead OR Management approval ✅ (2/1)

**Bypass rule:** Triggered (2+ Team Lead approvals (Tier 1 exception)). This PR is approved regardless of tier.

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