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/queries.go

@@ -149,6 +149,13 @@ LIMIT ? OFFSET ?`
 )`
 )
 
+// selectOtherSourcesTitles lists the uuid, title and latest_release_date of
+// the opportunities not from one source whose latest_release_date is in
+// [from, before); it walks the latest_release_date index.
+const selectOtherSourcesTitles = `
+SELECT uuid, title, latest_release_date FROM opportunities
+WHERE source <> ? AND latest_release_date >= ? AND latest_release_date < ?`
+
 // ---------- opportunity_releases ----------
 
 // insertRelease stores a release; 0 rows affected means it was already