Admin CSV export (sources snapshot + audience time-series)

Per Codex v1 — boring-in-the-best-way: inspect/archive operational data outside
the app. Admin-gated, Python csv module, text/csv + attachment disposition.

* GET /api/admin/export/sources.csv — current-state snapshot per source: name,
  feed/homepage, status, visible, served/accepted/total, acceptance/duplicate/
  accepted-dup/image-coverage %, last success/error, retry-after, review.
* GET /api/admin/export/audience.csv?days= — summary block (visitors, returning,
  accounts, feedback, shares) + a blank line + the daily visits/opens series;
  range applies to audience, sources is a snapshot.
* source_health now also returns feed_url/homepage. Small download links on the
  Sources + Audience tabs.

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

goodnews/queries.py

@@ -295,7 +295,7 @@ def source_health(conn: sqlite3.Connection) -> list[dict]:
     rows = conn.execute(
         """
         SELECT
-            s.id, s.name, s.default_category AS category, s.active,
+            s.id, s.name, s.feed_url, s.homepage_url, s.default_category AS category, s.active,
             s.status, s.content_visible, s.retry_after_at,
             s.consecutive_failures AS failures, s.review_flag, s.review_reason,
             s.poll_interval_minutes AS interval_minutes,