feat(sql): add problem embedding vector index

backend/migrations/0005_problem_embedding.surql

@@ -0,0 +1,6 @@
+-- TarninaHub — migration 0005: problem embeddings for semantic matchmaking
+-- Idempotent, additive. Mirrors the innovation vector index.
+
+DEFINE FIELD IF NOT EXISTS embedding ON TABLE problem TYPE option<array<float>>;
+DEFINE INDEX IF NOT EXISTS problem_embedding ON TABLE problem FIELDS embedding
+  HNSW DIMENSION 1536 DIST COSINE TYPE F32;