In-feed Back button + clickable source on the article page

* Back button on feed views: drilling into a tag or source from a card now
  remembers where you came from (a small history stack), and a "← Back" appears
  in the view header to return there — chains of drill-ins included. Top-level
  nav (rail/bottom bar) resets the history.
* Article page: the source name is now a link into that source's in-app feed
  (/?source=<id>); the SPA reads the param on load and opens the source view
  (label falls back to the loaded feed's source name). Completes the
  "cards-only v1" — source is clickable on /a/ too now.

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

goodnews/api.py

@@ -720,7 +720,7 @@ def create_app() -> FastAPI:
         with get_conn() as conn:
             row = conn.execute(
                 "SELECT a.id, a.title, a.description, a.image_url, a.canonical_url, "
-                "a.duplicate_of, src.name AS source_name, s.reason_text, s.accepted, "
+                "a.duplicate_of, a.source_id, src.name AS source_name, s.reason_text, s.accepted, "
                 "(SELECT group_concat(t.tag) FROM article_tags t WHERE t.article_id = a.id) AS tags "
                 "FROM articles a JOIN sources src ON src.id = a.source_id "
                 "LEFT JOIN article_scores s ON s.article_id = a.id WHERE a.id = ?",