-- Polls are no longer a standalone entity in the UX: every poll is attached
-- to a dispatch. We keep the pulses + votes tables (vote uniqueness, status
-- derivation, admin history) and add a nullable FK from dispatches.
--
-- ON DELETE SET NULL — if an attached pulse is hard-deleted, the dispatch
-- survives without a poll rather than vanishing with it.

ALTER TABLE dispatches ADD COLUMN pulse_id INTEGER REFERENCES pulses(id) ON DELETE SET NULL;

CREATE INDEX idx_dispatches_pulse ON dispatches(pulse_id) WHERE pulse_id IS NOT NULL;