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>

goodnews/queries.py

@@ -25,6 +25,7 @@ _ARTICLE_COLUMNS = f"""
     a.canonical_url,
     a.published_at,
     a.image_url,
+    a.source_id,
     src.name AS source_name,
     s.topic,
     s.flavor,
@@ -57,6 +58,7 @@ def feed(
     max_cortisol: int | None = None,
     max_ragebait: int | None = None,
     tag: str | None = None,
+    source_id: int | None = None,
     sort: str = "ranked",
 ) -> list[dict]:
     """Return articles with categorical filters applied in SQL.
@@ -106,6 +108,9 @@ def feed(
     if tag:
         clauses.append("EXISTS (SELECT 1 FROM article_tags at WHERE at.article_id = a.id AND at.tag = ?)")
         params.append(tag.lower())
+    if source_id:
+        clauses.append("a.source_id = ?")
+        params.append(source_id)
 
     where = "WHERE " + " AND ".join(clauses)
     params.extend([limit, offset])