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_source_eusupply.go

@@ -0,0 +1,62 @@
+package config
+
+import (
+	"errors"
+	"reflect"
+	"time"
+)
+
+// JSCKSourceEUSupply is the EU Supply source's configuration key.
+const JSCKSourceEUSupply string = "eu_supply"
+
+// JSCSourceEUSupply configures scraping the public tenders on EU Supply's
+// UK portal at BaseURL (see internal/eusupply). The client waits a random
+// time from RequestMinGap to RequestMaxGap after each request; both must be
+// greater than 0, as the site's own limit isn't published, and a
+// RequestMaxGap less than RequestMinGap is read as RequestMinGap.
+type JSCSourceEUSupply struct {
+	JSCSourcesSource
+	BaseURL       string        `cfg:"base_url"        default:"https://uk.eu-supply.com"`
+	RequestMinGap time.Duration `cfg:"request_min_gap" default:"2s"`
+	RequestMaxGap time.Duration `cfg:"request_max_gap" default:"10s"`
+}
+
+// ---------- Section methods (see Section) ----------
+
+// Map implements Section.
+func (c JSCSourceEUSupply) Map() map[string]any { return mapOf(c) }
+
+// Flatten implements Section.
+func (c JSCSourceEUSupply) Flatten() map[string]any {
+	return flattenOf(JSCKSourceEUSupply, c)
+}
+
+// TypeOf implements Section.
+func (c JSCSourceEUSupply) TypeOf(key string) (reflect.Kind, error) {
+	return typeOf(c, key)
+}
+
+// ValueOf implements Section.
+func (c JSCSourceEUSupply) ValueOf(key string) (any, error) {
+	return valueOf(c, key)
+}
+
+// SetValue implements Section.
+func (c *JSCSourceEUSupply) SetValue(key string, value any) error {
+	return setValue(c, key, value)
+}
+
+// ---------- Section specific methods ----------
+
+// validate checks values before they are stored (see validator), as
+// JSCSourcesSource's does, which this one hides. request_min_gap and
+// request_max_gap must be greater than 0.
+func (c *JSCSourceEUSupply) validate(key string, value any) error {
+	if (key == "request_min_gap" || key == "request_max_gap") &&
+		value.(time.Duration) <= 0 {
+		return errors.New(
+			"config: request_min_gap and request_max_gap must be greater than 0, as EU Supply publishes no rate limit",
+		)
+	}
+	return c.JSCSourcesSource.validate(key, value)
+}