feat: Add SLD endpoints, add date_to to vector_with_kato

migrations/00009_add_period_to_dataset_observations.sql

@@ -0,0 +1,15 @@
+-- +goose Up
+-- Optional period end date, so a year_column mapping can represent a date
+-- range (date..date_to) instead of a single point in time. NULL preserves the
+-- pre-existing point-in-time behavior; the unique index treats a NULL date_to
+-- as equal to date itself so point observations keep deduplicating exactly as
+-- before.
+ALTER TABLE dataset_observations ADD COLUMN date_to DATE;
+
+DROP INDEX uq_dataset_observations;
+CREATE UNIQUE INDEX uq_dataset_observations ON dataset_observations (dataset_id, kato_code, date, COALESCE(date_to, date));
+
+-- +goose Down
+DROP INDEX uq_dataset_observations;
+ALTER TABLE dataset_observations DROP COLUMN date_to;
+CREATE UNIQUE INDEX uq_dataset_observations ON dataset_observations (dataset_id, kato_code, date);