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/config/conf_sections_test.go

@@ -288,6 +288,35 @@ func TestConcreteSourceSections(t *testing.T) {
 	}
 }
 
+func TestEUSupplySection(t *testing.T) {
+	conf := defaultConfig(t)
+	got := conf.Sources.EUSupply
+	if got.Enabled || got.Interval != 10*time.Minute ||
+		got.BaseURL != "https://uk.eu-supply.com" ||
+		got.RequestMinGap != 2*time.Second ||
+		got.RequestMaxGap != 10*time.Second {
+		t.Errorf("defaults = %+v", got)
+	}
+
+	c := JSCSourceEUSupply{}
+	if err := c.SetValue("request_min_gap", "5s"); err != nil ||
+		c.RequestMinGap != 5*time.Second {
+		t.Errorf("SetValue(request_min_gap, 5s): %v, %v", err, c.RequestMinGap)
+	}
+	for _, key := range []string{"request_min_gap", "request_max_gap"} {
+		if err := c.SetValue(key, "0s"); err == nil {
+			t.Errorf("SetValue(%s, 0s) succeeded", key)
+		}
+	}
+	// Its own validate must still check interval, as JSCSourcesSource's does.
+	if err := c.SetValue("interval", "0s"); err == nil {
+		t.Error("SetValue(interval, 0s) succeeded")
+	}
+	if v, err := c.ValueOf("base_url"); err != nil || v != "" {
+		t.Errorf("ValueOf(base_url) = %v, %v", v, err)
+	}
+}
+
 func TestSourcesSection(t *testing.T) {
 	var c JSCSources
 	if err := c.SetValue("fts_mock.api_root", "http://mock"); err != nil {
@@ -317,8 +346,8 @@ func TestSourcesSection(t *testing.T) {
 	if flat["sources.fts_mock.api_root"] != "http://mock" {
 		t.Errorf("Flatten missing sources.fts_mock.api_root: %v", flat)
 	}
-	if len(flat) != 12 {
-		t.Errorf("Flatten has %d keys; want 12: %v", len(flat), flat)
+	if len(flat) != 17 {
+		t.Errorf("Flatten has %d keys; want 17: %v", len(flat), flat)
 	}
 
 	if err := c.SetValue("skip_rules_file", "rules.json"); err != nil ||