Add pytest integration tests that run EXPLAIN against real Postgres schema
to detect phantom column/table references before PR merge. 12 EXPLAIN test
cases covering domrf_kn_objects, ekburg_construction_permits,
mv_layout_velocity, domrf_kn_flats, objective_* and cad geo tables.
Tests skip without TEST_DATABASE_URL; CI integration is a follow-up.
Pre-empts phantom column bugs caught manually on PR #196 (geom_3857)
and PR #213 (units_count).
Closes#197