Supervised source candidates: stage, list, promote, reject

- New source_candidates staging table (status suggested/quarantined/rejected/
  promoted, preview_json snapshot) so untrusted/suggested feeds stay out of the
  real ingestion path until reviewed.
- sources.py: save_candidate (re-preview never revives a curator's rejection),
  list_candidates, reject_candidate, promote_candidate (copies into sources,
  inactive by default — active on approval; never automatic).
- CLI: suggest-source / list-candidates / promote-candidate / reject-candidate.
- API: read-only GET /api/candidates (writes stay CLI-only — no unauthenticated
  public write surface yet).
- Fix deprecated ElementTree truth-value test in _parse_rss.
- Tests: candidate lifecycle (save/list/promote/reject, status preservation,
  name derivation) — 51 total.

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

goodnews/feeds.py

@@ -321,7 +321,9 @@ def insert_article(conn: sqlite3.Connection, source: sqlite3.Row, item: FeedItem
 
 
 def _parse_rss(root: ET.Element) -> list[FeedItem]:
-    channel = _first_child(root, "channel") or root
+    channel = _first_child(root, "channel")
+    if channel is None:
+        channel = root
     language = _first_text(channel, "language")
     items = [element for element in root.iter() if _local_name(element.tag) == "item"]
     parsed = []