feat(api): pgvector nearest-neighbor substitution search

[rvnminers-A-and-N]

Implement nearest-neighbor substitution search over the flavor-space embeddings using pgvector ("approved ingredients closest to this one"). Depends on M4 embeddings + M5 pgvector.

Acceptance

• Embeddings indexed in pgvector
• /substitutes returns ranked nearest neighbors
• Results carry similarity scores

[rvnminers-A-and-N]

Track-A core landed (PRs #46/#47): predict.substitute() — Tanimoto/Morgan nearest-neighbor over the labeled set, each neighbor tagged with its known tastes — and the demo /api/neighbors now delegates to it. This issue stays open for the M6 product version: the .NET API doing this as a pgvector ANN query over the same fingerprints at scale.