Add topic enrichment and metadata merge rules for publication

[voidrot]

Summary

Define and implement how model-suggested topics, tags, and other metadata are merged with source-promoted metadata before publication.

Why this work is needed

The model should be able to suggest richer publication metadata, but that metadata needs explicit precedence and grounding rules so publish remains deterministic and trustworthy.

Scope

• Define merge and precedence rules for metadata.
• Keep provenance and visibility grounded in source data and server policy.
• Support additional topics or tags where justified.
• Add tests for merge behavior and conflict handling.

Out of scope

• Retrieval scoring changes.
• Frontend metadata editing UX.

Acceptance criteria

• Metadata merge rules are explicit and implemented.
• Conflicts between source-promoted and model-suggested metadata are handled predictably.
• The behavior is covered by tests.

Dependencies

• Depends on Epic: Compiler multi-document publishing and metadata enrichment #62.
• Depends on Update draft and review flow for multi-document and topic proposals #74.
• Depends on Refactor publish to emit multiple live documents #76.