feat(F-002): database foundation with migrations and dev compose

- node-pg-migrate + pg: baseline migration (extensions, app_meta) with working down
- src/infrastructure/db fail-fast pool and typed query helper
- docker-compose: postgres:16-alpine + redis:7-alpine with one-command up
- table naming convention <module>_<table> documented in README
- integration tests (6) against real PostgreSQL; strict identifier validation
  for test DDL after security-gate hardening round
- deps justified in spec/tech.md; all gates approved; verify.sh green

project/.env.example

@@ -0,0 +1,4 @@
+# Copy to .env for local development. Real .env is gitignored.
+# Matches docker-compose.yml dev credentials (dev-only, never reuse elsewhere).
+DATABASE_URL=postgres://mdv:mdv_dev_only@localhost:5432/mercadodevida
+TEST_DATABASE_URL=postgres://mdv:mdv_dev_only@localhost:5432/mercadodevida_test