Added an EU Supply source, scraping the UK portal's public tenders

internal/eusupply replaces the old Node scraper (eu-supply.js) for
uk.eu-supply.com. The site has no API, so it posts the public search
form, which now needs the form's current defaults and reads the HTML
with golang.org/x/net/html. Each tender not stored yet has its own
page read too, for its description, contract type, CPV codes and
buyer address. Tenders are stored with the source "EU Supply - UK"
as eusupply-uk-<tender id>, with one release holding the tender as
read.

The site publishes no rate limit, so the client sets its own: one
request at a time, with a random wait between
sources.eu_supply.request_min_gap and request_max_gap (2s to 10s).
The source has the same 7-day backfill as Find a Tender's.

Most of its tenders are also on Find a Tender, so a new one is stored
Hidden and Skipped when another source's opportunity released within
30 days has a title at least 90% alike, and its buyer or description
matches too (80% of the shorter one's significant words).

internal/store/store.go

@@ -1041,6 +1041,37 @@ func (s *Store) LatestReleaseDate(
 	return latest, nil
 }
 
+// OpportunityTitle is an opportunity's UUID, title and latest release date,
+// as ListOtherSourcesTitles lists them.
+type OpportunityTitle struct {
+	UUID              string    `db:"uuid"`
+	Title             string    `db:"title"`
+	LatestReleaseDate time.Time `db:"latest_release_date"`
+}
+
+// ListOtherSourcesTitles returns the UUID, title and latest release date of
+// every opportunity not from source whose latest release is from from
+// (inclusive) to before (exclusive), so a fetcher can find the ones a new
+// opportunity repeats.
+func (s *Store) ListOtherSourcesTitles(
+	ctx context.Context, source string, from, before time.Time,
+) ([]OpportunityTitle, error) {
+	titles := []OpportunityTitle{}
+	err := s.db.SelectContext(
+		ctx,
+		&titles,
+		selectOtherSourcesTitles,
+		source,
+		from.UTC(),
+		before.UTC(),
+	)
+	if err != nil {
+		return nil, fmt.Errorf("listing titles of sources but %s: %w", source, err)
+	}
+
+	return titles, nil
+}
+
 // ReleaseExists reports whether a release is already stored, so a fetcher
 // can skip converting it.
 func (s *Store) ReleaseExists(