feat(search): pgvector substrate — vector(384) + HNSW for semantic search

Move semantic_search_notes off the full-table Python cosine scan onto a native
pgvector column: indexed ORDER BY embedding <=> :q LIMIT k (HNSW, cosine).
Migration 0067 enables the extension, converts the JSONB embedding column to
vector(384) (stale-dim rows dropped and regenerated by the startup backfill),
and builds the HNSW cosine index. Postgres image moves postgres:16-alpine ->
pgvector/pgvector:pg17 across prod, quickstart, and CI.

Scribe: project 2, milestone 93, task 1031.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xz4j1H7pjYSjKsEpgcNH5E

docker-compose.quickstart.yml

@@ -35,7 +35,8 @@ services:
       start_period: 30s
 
   db:
-    image: postgres:16-alpine
+    # pgvector image (PG17) — bundles the `vector` extension (migration 0067).
+    image: pgvector/pgvector:pg17
     stop_grace_period: 120s
     volumes:
       - pgdata:/var/lib/postgresql/data