Source feeds: click a source to see its publication feed

Click a source name on any card → a feed of just that source's articles,
newest-first, still accepted / non-duplicate / boundary-filtered (the calm
promise isn't bypassed). A natural way to follow a publication's feel.

* queries.feed + /api/feed: source_id filter; Article output gains source_id.
* Frontend: source label is a button → transient 'source:<id>' view (like
  'tag:<slug>'), rendered in the feed grid with Load more, header = source name.
* Ad-hoc, not a pinned lane. Foundation for a future source page (metadata) +
  Follow; shareable /source/<slug> route and source_view analytics come then.

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

tests/test_feed_sort.py

@@ -25,3 +25,16 @@ def test_latest_sorts_strictly_by_recency(tmp_path):
 
     latest = [a["id"] for a in queries.feed(c, sort="latest")]
     assert latest == [2, 1, 3]  # newest → oldest, regardless of insert order
+
+
+def test_feed_source_id_filters_to_one_source(tmp_path):
+    c = connect(str(tmp_path / "t.db")); init_db(c)
+    c.execute("INSERT INTO sources (id,name,feed_url) VALUES (1,'A','http://a/f'),(2,'B','http://b/f')")
+    for aid, sid in [(1, 1), (2, 2), (3, 1)]:
+        c.execute("INSERT INTO articles (id,source_id,canonical_url,title,url_hash,published_at) "
+                  "VALUES (?,?,?,?,?,'2026-01-01T00:00:00')", (aid, sid, f"u{aid}", f"T{aid}", f"h{aid}"))
+        c.execute("INSERT INTO article_scores (article_id,accepted) VALUES (?,1)", (aid,))
+    c.commit()
+    ids = {a["id"] for a in queries.feed(c, source_id=1)}
+    assert ids == {1, 3}  # only source A
+    assert all(a["source_id"] == 1 for a in queries.feed(c, source_id=1))