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/store.go

@@ -776,10 +776,14 @@ func (s *Store) LatestChangeBatch(ctx context.Context) (string, error) {
 // those batches, to give it next time. With no newer batches, it returns no
 // opportunities and after. An unknown hash is ErrUnknownBatch.
 //
+// Unless releasedFrom is zero, only opportunities whose latest release is on
+// or after it are returned (as OpportunityFilter.ReleasedFrom); the hash
+// returned is the same either way.
+//
 // Batches recorded while it runs are left for next time, so none is missed:
 // only those up to the newest batch when it starts are read.
 func (s *Store) ListChangedOpportunities(
-	ctx context.Context, after string,
+	ctx context.Context, after string, releasedFrom time.Time,
 ) ([]types.Opportunity, string, error) {
 	var since changeBatch
 	err := s.db.GetContext(ctx, &since, selectChangeBatchByHash, after)
@@ -796,14 +800,16 @@ func (s *Store) ListChangedOpportunities(
 		return nil, "", fmt.Errorf("reading the latest change batch: %w", err)
 	}
 
+	query := selectOpportunitiesChangedBetween
+	args := []any{since.CreatedAt, latest.CreatedAt}
+	if !releasedFrom.IsZero() {
+		query += "\nAND " + filterOpportunityReleasedFrom
+		args = append(args, releasedFrom.UTC())
+	}
+	query += orderOpportunitiesChanged
+
 	var rows []opportunityRow
-	err = s.db.SelectContext(
-		ctx,
-		&rows,
-		selectOpportunitiesChangedBetween,
-		since.CreatedAt,
-		latest.CreatedAt,
-	)
+	err = s.db.SelectContext(ctx, &rows, query, args...)
 	if err != nil {
 		return nil, "", fmt.Errorf("listing changed opportunities: %w", err)
 	}