Track 3: read-only source preview (vet a feed before adding)

- feeds.preview_feed(): fetch + score a sample WITHOUT persisting; returns
  freshness, acceptance rate, cortisol/ragebait/PR averages, and example
  accepted/rejected items. With an LLM client it also returns topic/flavor mix
  and the model's (accurate) acceptance view.
- CLI 'preview-source URL [--sample] [--classify]'.
- API 'GET /api/source-preview?url=&sample=&classify=' with an http(s)-only
  guard (SSRF note left for go-public hardening).
- Site 'Suggest a source' panel with Quick check (heuristic, instant) and Deep
  check (model, accurate), rendered DOM-safely.
- Tests: network-free preview_feed tests via monkeypatched fetch (45 total).
- README documents the command, endpoint, and updated roadmap.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_preview.py

@@ -0,0 +1,36 @@
+import goodnews.feeds as feeds
+
+RSS = b"""<?xml version="1.0"?>
+<rss><channel>
+  <item>
+    <title>Volunteers restore creek and rescue stranded wildlife</title>
+    <description>A hopeful recovery effort</description>
+    <link>http://example.com/1</link>
+    <pubDate>Sat, 30 May 2026 10:00:00 GMT</pubDate>
+  </item>
+  <item>
+    <title>Quarterly tax filing deadline reminder</title>
+    <description>routine notice</description>
+    <link>http://example.com/2</link>
+    <pubDate>Sat, 30 May 2026 09:00:00 GMT</pubDate>
+  </item>
+</channel></rss>"""
+
+
+def test_preview_feed_scores_sample_without_network(monkeypatch):
+    monkeypatch.setattr(feeds, "fetch_feed", lambda url, **kw: RSS)
+    p = feeds.preview_feed("http://example.com/feed")
+
+    assert p["sampled"] == 2
+    assert p["classified"] is False
+    assert 0.0 <= p["acceptance_rate"] <= 1.0
+    assert p["accepted"] >= 1  # the restore/rescue story should pass the heuristic
+    assert p["newest_published"] is not None
+    assert isinstance(p["topic_mix"], dict) and p["topic_mix"] == {}  # empty without a model
+    assert all("reason" in ex and "title" in ex for ex in p["examples_rejected"])
+
+
+def test_preview_feed_respects_sample_cap(monkeypatch):
+    monkeypatch.setattr(feeds, "fetch_feed", lambda url, **kw: RSS)
+    p = feeds.preview_feed("http://example.com/feed", sample=1)
+    assert p["sampled"] == 1