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.go

@@ -0,0 +1,109 @@
+package router
+
+import "strings"
+
+// Feed queries — "что нового в лентах?", "что нового по технологиям?"
+// (Vikunja #258).
+//
+// Deterministic matching, like the calendar, plan and habit matchers above it:
+// the LLM router says this is a query, and this decides whether it is a question
+// about the feeds. A model deciding that would occasionally answer "что нового?"
+// out of world knowledge, which is the one thing a feed reader exists to avoid.
+
+// FeedQuery — a parsed "what's new" question. Category is the topic he named
+// ("технологии"), empty when he asked about the feeds in general.
+type FeedQuery struct {
+	Category string
+}
+
+// feedNouns — the words that make a question about the feeds themselves.
+var feedNouns = []string{
+	"лента", "ленте", "ленты", "лентах", "лентам",
+	"новости", "новостей", "новостях", "новостям",
+	"новое", "нового", "новенького",
+	"feed", "feeds", "news", "headlines",
+}
+
+// newnessMarkers — the "что нового" half. "нового" alone is in feedNouns
+// because it carries the question on its own ("что нового?"); a bare "лента"
+// needs the ask, which is what askMarkers below is for.
+var askMarkers = []string{
+	"что", "какие", "какое", "расскажи", "почитай", "прочитай", "покажи",
+	"what", "any", "tell", "show", "read",
+}
+
+// ParseFeedQuery reports whether an utterance asks what is new in the feeds, and
+// which topic if it names one after "по"/"о"/"про"/"about".
+//
+// Both a feed noun and an ask are required. "у меня новая лента в инстаграме" is
+// a statement and must not be read as a request to recite headlines.
+func ParseFeedQuery(text string) (FeedQuery, bool) {
+	toks := planTokens(text)
+	noun, ask := false, false
+	for _, t := range toks {
+		for _, n := range feedNouns {
+			if t == n {
+				noun = true
+				break
+			}
+		}
+		for _, a := range askMarkers {
+			if t == a {
+				ask = true
+				break
+			}
+		}
+	}
+	if !noun || !ask {
+		return FeedQuery{}, false
+	}
+	return FeedQuery{Category: feedCategory(toks)}, true
+}
+
+// categoryPreps — the prepositions a topic follows. Russian marks the topic with
+// a preposition ("по технологиям", "про политику"), so the word after one is the
+// category; there is no stemming here, and the match against the configured
+// category is a prefix comparison for exactly that reason.
+var categoryPreps = map[string]bool{"по": true, "о": true, "об": true, "про": true, "about": true, "on": true}
+
+func feedCategory(toks []string) string {
+	for i, t := range toks {
+		if categoryPreps[t] && i+1 < len(toks) {
+			next := toks[i+1]
+			// "по новостям" names no topic, it repeats the noun.
+			for _, n := range feedNouns {
+				if next == n {
+					return ""
+				}
+			}
+			return next
+		}
+	}
+	return ""
+}
+
+// CategoryMatches reports whether a note's text plausibly belongs to the
+// category he named. Russian inflects the topic ("технологиям" vs the configured
+// "технологии"), and there is no stemmer in this repo, so the comparison is on a
+// common prefix — long enough that "полит" and "погод" stay apart, short enough
+// to survive a case ending.
+func CategoryMatches(text, category string) bool {
+	if category == "" {
+		return true
+	}
+	stem := categoryStem(category)
+	if stem == "" {
+		return false
+	}
+	return strings.Contains(strings.ToLower(text), stem)
+}
+
+// categoryStem cuts a word down to the part inflection leaves alone. 5 runes is
+// the compromise: shorter words are used whole.
+func categoryStem(word string) string {
+	r := []rune(strings.ToLower(strings.TrimSpace(word)))
+	if len(r) > 5 {
+		r = r[:5]
+	}
+	return string(r)
+}

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")
+	}
+}