Read RSS and Atom feeds, and speak about them only when asked (#258)

internal/rss parses RSS 2.0 and Atom, and polls each configured feed on its own
interval; internal/webfetch is the one door either of them uses to touch the
network. The poller writes items as notes with source "rss:<feed>" and nothing
else: the answer path reads them back when he asks "что нового в лентах?", and
nothing is announced on arrival. A feed that dispatched would be a nag, which is
why the plan's breaking-news rule was left out rather than built.

webfetch is where the limits live, as code rather than a paragraph: http(s)
only, an allowlist (the configured feeds' hosts) and a denylist, a 2 MiB body
cap, a 3-redirect cap, one request per host per second, and a refusal to connect
to any private address — checked in the dialer's Control hook so it holds for
every resolved address and every redirect hop, not just for a literal IP.

Off unless configured: no "feeds" block, no poller, no outbound request. How far
a feed was read is a config fact (rss:latest:<name>), so a restart does not
re-note yesterday's headlines.

internal/router/feeds_test.go

@@ -0,0 +1,48 @@
+package router
+
+import "testing"
+
+func TestParseFeedQuery(t *testing.T) {
+	cases := []struct {
+		text     string
+		ok       bool
+		category string
+	}{
+		{"что нового в лентах?", true, ""},
+		{"что нового?", true, ""},
+		{"какие новости?", true, ""},
+		{"что нового по технологиям?", true, "технологиям"},
+		{"расскажи новости про политику", true, "политику"},
+		{"что нового по новостям", true, ""},
+		{"what's new in the feeds?", true, ""},
+		{"any news about kubernetes", true, "kubernetes"},
+		// Statements, not requests.
+		{"у меня новая лента в инстаграме", false, ""},
+		{"новости меня утомили", false, ""},
+		{"напомни полить цветы", false, ""},
+		{"", false, ""},
+	}
+	for _, c := range cases {
+		q, ok := ParseFeedQuery(c.text)
+		if ok != c.ok {
+			t.Errorf("ParseFeedQuery(%q) ok = %v, want %v", c.text, ok, c.ok)
+			continue
+		}
+		if ok && q.Category != c.category {
+			t.Errorf("ParseFeedQuery(%q) category = %q, want %q", c.text, q.Category, c.category)
+		}
+	}
+}
+
+func TestCategoryMatches(t *testing.T) {
+	// The inflected form he says must match the form the config spells.
+	if !CategoryMatches("Новый релиз [технологии]", "технологиям") {
+		t.Error("inflected category did not match")
+	}
+	if CategoryMatches("Новый релиз [технологии]", "политику") {
+		t.Error("unrelated category matched")
+	}
+	if !CategoryMatches("anything", "") {
+		t.Error("an empty category must match everything")
+	}
+}