Enforce user_embeddings provenance ownership consistency

[AnthonyRonning]

Summary

Follow-up hardening from PR #161.

user_embeddings currently validates provenance shape (user_message_id vs assistant_message_id, conversation_id presence, etc.), but it does not enforce at the database layer that the referenced message rows belong to the same user_id / conversation_id as the embedding row.

Why this is worth tracking

• Current write paths appear safe, so this did not block the schema-sync PR.
• A bad future write path could still create cross-user or cross-conversation provenance links.
• This is the kind of invariant that is easy to assume in code and hard to notice once violated.

Affected migration

• migrations/2026-02-10-020612_user_embeddings

Follow-up direction

• Add DB-level ownership validation for user_message_id / assistant_message_id / conversation_id.
• Composite FKs may work if the referenced tables expose the needed uniqueness; otherwise use a trigger-based validation approach.
• Preserve the existing Feb/Mar migration lineage by addressing this in a new forward migration rather than editing the historical migration files.