Add exclusion constraints for driver, tractor, and trailer assignment overlap prevention

[emoss08]

Add exclusion constraints to prevent double-booking of drivers, tractors, and trailers. Application-layer validation alone is insufficient — concurrent requests can pass validation simultaneously and both commit.

Requires

btree_gist extension (covered in TRE-164).

Constraints to add

Driver assignments

ALTER TABLE driver_assignments
    ADD CONSTRAINT no_overlapping_driver_assignments
    EXCLUDE USING gist (
        driver_id WITH =,
        int8range(start_time, end_time, '[]') WITH &&
    );

Tractor assignments

ALTER TABLE tractor_assignments
    ADD CONSTRAINT no_overlapping_tractor_assignments
    EXCLUDE USING gist (
        tractor_id WITH =,
        int8range(start_time, end_time, '[]') WITH &&
    );

Trailer assignments

ALTER TABLE trailer_assignments
    ADD CONSTRAINT no_overlapping_trailer_assignments
    EXCLUDE USING gist (
        trailer_id WITH =,
        int8range(start_time, end_time, '[]') WITH &&
    );

Notes

• Table and column names above are illustrative — adjust to match actual schema
• These constraints enforce at the database level what is currently only validated in the application
• Pairs well with the no_overlapping_periods constraint added in TRE-163 for fiscal periods