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/eusupply/details.go

@@ -0,0 +1,108 @@
+package eusupply
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+
+	"golang.org/x/net/html"
+)
+
+// ErrNoDetails means a tender's page had none of the labelled fields the
+// public page always has: the page has changed, or isn't a tender's.
+var ErrNoDetails = errors.New("eusupply: no tender details on the page")
+
+// Details reads tender's own page and fills in its description, contract
+// type, CPV codes, buyer address and number of lots, and sets HasDetails.
+func (c *Client) Details(ctx context.Context, tender *Tender) error {
+	body, err := c.get(ctx, tenderPath(tender.PID))
+	if err != nil {
+		return fmt.Errorf("reading tender %s: %w", tender.ID, err)
+	}
+
+	if err := parseDetails(body, tender); err != nil {
+		return fmt.Errorf("reading tender %s: %w", tender.ID, err)
+	}
+
+	return nil
+}
+
+// parseDetails reads a tender's page into tender. Each field is a
+// "ctm-content-label" div ("CPV codes:") followed by a <p> holding its
+// value.
+func parseDetails(body []byte, tender *Tender) error {
+	document, err := html.Parse(bytes.NewReader(body))
+	if err != nil {
+		return err
+	}
+
+	values := make(map[string]*html.Node)
+	labels := findAll(document, hasClass("div", "ctm-content-label"))
+	for _, label := range labels {
+		value := nextElement(label)
+		if value == nil || value.Data != "p" {
+			continue
+		}
+		name := strings.TrimSuffix(text(label), ":")
+		if _, found := values[name]; !found {
+			values[name] = value
+		}
+	}
+	if len(values) == 0 {
+		return ErrNoDetails
+	}
+
+	if value := values["Detailed description"]; value != nil {
+		tender.Description = strings.TrimSpace(rawText(value))
+	}
+	if value := values["Type of contract"]; value != nil {
+		tender.ContractType = text(value)
+	}
+	if value := values["CPV codes"]; value != nil {
+		tender.CPVCodes = parseCPVCodes(value)
+	}
+	if value := values["Buyer"]; value != nil {
+		tender.BuyerAddress = buyerAddress(value)
+	}
+	if deadline := values["Response deadline (UK time)"]; deadline != nil &&
+		tender.Deadline == nil {
+		if parsed, err := parseUKTime(text(deadline)); err == nil {
+			tender.Deadline = &parsed
+		}
+	}
+	tender.Lots = len(findAll(document, hasClass("div", "package-container")))
+	tender.HasDetails = true
+
+	return nil
+}
+
+// parseCPVCodes reads the CPV codes listed in value, one per <span>: the
+// code, two non-breaking spaces, then its description.
+func parseCPVCodes(value *html.Node) []CPVCode {
+	var codes []CPVCode
+	for _, span := range findAll(value, isTag("span")) {
+		code, description, _ := strings.Cut(text(span), " ")
+		if code == "" {
+			continue
+		}
+		codes = append(codes, CPVCode{
+			Code:        code,
+			Description: description,
+		})
+	}
+	return codes
+}
+
+// buyerAddress reads the buyer's name and address, one line each, leaving
+// out the "View profile" link.
+func buyerAddress(value *html.Node) []string {
+	var address []string
+	for _, line := range lines(value) {
+		if line != "View profile" {
+			address = append(address, line)
+		}
+	}
+	return address
+}