Database doctor

Structural checks for Postgres drift (schemas, key tables/views, single fn_alert_candidates overload with 23 parameters, selectable calibration export views).

make db-doctor

DB_DOCTOR_WARN_ONLY=1 prints failures but exits 0.

Requires a reachable database (POSTGRES_* / PSYCOPG_CONNINFO) and applied migrations. Typical healthy rows include fn_alert_candidates:single_overload yes and param_count matching expected=23.