Source Retire lifecycle (Phase 1: status + content_visible, active mirrored)

Per Codex's plan — introduce a lifecycle without a risky "change the source of
truth everywhere" moment.

* Schema: sources.status (active|paused|retired) + content_visible; migration
  backfills status from active (active=1→active, else paused), content_visible=1.
* `active` is kept as a SYNCED MIRROR: status active→active=1, paused/retired→0,
  so the scheduler/CLI/legacy code keep working unchanged.
* Retire stops polling but keeps articles visible (non-destructive). Hiding is a
  separate, reversible lever: content_visible=0 drops a source's articles from
  the public feed + brief (read AND build), behind a confirm. Personal saved/
  history are untouched.
* API: /sources/{id}/status (validates, mirrors active) + /visibility, replacing
  /active. source_health returns status + content_visible.
* Admin: status column (active/paused/retired + "hidden"), Retired filter,
  Pause/Resume · Retire/Restore · Hide/Show actions.

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

goodnews/briefs.py

@@ -139,6 +139,7 @@ def _candidate_articles(
         JOIN sources src ON src.id = a.source_id
         JOIN article_scores s ON s.article_id = a.id
         WHERE s.accepted = 1
+          AND src.content_visible = 1
           AND a.duplicate_of IS NULL
           AND date(COALESCE(a.published_at, a.discovered_at)) <= date(?)
           AND date(COALESCE(a.published_at, a.discovered_at)) > date(?, '-' || ? || ' days')