feat: add PostgreSQL query-store foundation (#148)

[atomantic]

Summary

• add an idempotent PostgreSQL baseline for the complete derived query-store model using native boolean, timestamptz, jsonb, identity, and tsvector/GIN types
• add a lazy async pg service with named-parameter compilation, availability checks, transactional schema initialization, and deterministic pool shutdown
• package both SQL schemas, pass DATABASE_URL through PM2 without storing credentials, and document the staged rollout
• keep the current SQLite/JSON runtime unchanged until later Migrate query store from SQLite (better-sqlite3) to PostgreSQL #120 slices migrate writers and readers

Test plan

• npm run build
• full Vitest suite under the repository's compatible Node 24 SQLite ABI: 175 passed, 2 skipped
• npm test -- --run tests/unit (120 passed)
• SPARSETREE_TEST_DATABASE_URL='postgresql:///postgres?host=/tmp' npm test -- --run tests/integration/db/postgresSchema.spec.ts (2 passed against live PostgreSQL; temporary schema removed)
• npm run check:file-sizes
• git diff --check

Part of #120

Closes #148