[c20 follow-up] ADR-043 §8 startup backfill (steps 2-4)

[ohdearquant]

Deferred from PR #374 (c20). The migration creates the _embedding_models registry but does not:

• Insert rows from [[engines]] config
• Backfill existing vec_.embedding_model_id values
• Rebuild vec0 tables with CHECK NOT NULL on embedding_model_id
• Emit EmbeddingModelChanged events

Reference: codex_review_pr374.md round-1 MAJ-2.