Added a "Published in the last X days" filter, applied by the backend

GET /api/opportunities takes released_from, an RFC 3339 time, and lists
only the opportunities whose latest release is at or after it. It works
with paging and with changed_after, which keeps the same batch either
way; an invalid value is 400.

The frontend's filter defaults to 7 days, from midnight UK time, and
sends it with every load and catch-up, so a year of opportunities isn't
downloaded to show a week's. Changing the days loads the list again;
Clear filters goes back to 7.

internal/store/queries.go

@@ -313,7 +313,8 @@ ON CONFLICT DO NOTHING`
 
 // selectOpportunitiesChangedBetween reads, into opportunityRows, the
 // opportunities in the batches created after the first argument and at or
-// before the second, once each, ordered as ListOpportunities orders them.
+// before the second, once each. ListChangedOpportunities may add AND
+// clauses, then orderOpportunitiesChanged.
 const selectOpportunitiesChangedBetween = `
 SELECT t.*
 FROM opportunities t
@@ -322,5 +323,9 @@ WHERE t.uuid IN (
 	FROM opportunity_change_batch_opportunities bo
 	JOIN opportunity_change_batches b ON b.id = bo.batch_id
 	WHERE b.created_at > ? AND b.created_at <= ?
-)
+)`
+
+// orderOpportunitiesChanged ends selectOpportunitiesChangedBetween, ordering
+// as ListOpportunities orders.
+const orderOpportunitiesChanged = `
 ORDER BY t.latest_release_date DESC, t.uuid`