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/fetcher/fetcher.go

@@ -185,6 +185,18 @@ func (f *Fetcher) Run(ctx context.Context, since time.Time) error {
 			return runFTSMock(ctx, logger, &f.conf.FTSMock, f.save, since)
 		})
 	}
+	if f.conf.EUSupply.Enabled {
+		// A nil *store.Store in the interface would not be nil.
+		var lookup tenderStore
+		if f.store != nil {
+			lookup = f.store
+		}
+		start(euSupplySource, func(ctx context.Context, since time.Time) error {
+			return runEUSupply(
+				ctx, logger, &f.conf.EUSupply, lookup, f.save, since,
+			)
+		})
+	}
 	if f.conf.Sleeper.Enabled {
 		start("Sleeper", func(ctx context.Context, since time.Time) error {
 			return runSleeper(ctx, logger, &f.conf.Sleeper, since)