Fix glued-together RSS ledes; parse per-item language

Two ingestion changes:

- extractLede replaced HTML tags with empty string, so adjacent
  block tags like </p><p> fused words across paragraphs. Replace
  tags with a space and collapse whitespace.
- Pull each item's <language> tag (or dc:language) into FeedItem
  so the poller can filter on it. Politico Europe publishes the
  same story in en / fr / de side-by-side and we want to keep
  only one language per source.

internal/ingestion/parser_test.go

@@ -48,6 +48,11 @@ func TestExtractLede(t *testing.T) {
 			"It's a new era—one of change.",
 			"It's a new era\u2014one of change.",
 		},
+		{
+			"adjacent block tags inject a space",
+			"<p>...end of moments</p><p>Clarence B Jones, a former...</p>",
+			"...end of moments Clarence B Jones, a former...",
+		},
 		{
 			"tags and entities combined",
 			"<p>Oil prices rose &amp; gas fell by &gt;5%.</p>",