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/db.py

@@ -83,6 +83,19 @@ CREATE TABLE IF NOT EXISTS ingest_runs (
     error TEXT
 );
 
+CREATE TABLE IF NOT EXISTS source_candidates (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    feed_url TEXT NOT NULL UNIQUE,
+    homepage_url TEXT,
+    name TEXT,
+    status TEXT NOT NULL DEFAULT 'suggested',
+    preview_json TEXT,
+    notes TEXT,
+    last_previewed_at TEXT,
+    created_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    updated_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
 CREATE TABLE IF NOT EXISTS daily_briefs (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     brief_date TEXT NOT NULL UNIQUE,