Durable image quality: stop trusting feed thumbnails; cycle enriches Latest

Make "no blurry images" sustainable, not a one-off cleanup. RSS feed thumbnails
(~44% were ~90px) were stored at ingest and upscaled to mush, so new articles
would reintroduce them. Now image_url is filled ONLY by the quality-gated
og:image enrichment:

* insert_article no longer stores the feed image (was canonicalize_url(item...)).
* enrich_recent_images(): the cycle fetches a quality og:image for the newest
  accepted, imageless articles each run (bounded), keeping Latest photo-rich.
* Brief + on-open enrichment unchanged.

Net: every stored image is a validated, ≥450px og:image; the rest are clean
placeholders.

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

goodnews/feeds.py

@@ -323,7 +323,10 @@ def insert_article(conn: sqlite3.Connection, source: sqlite3.Row, item: FeedItem
                 description,
                 clean_text(item.author, max_len=250),
                 item.published_at,
-                canonicalize_url(item.image_url),
+                # Don't store the feed's image: RSS thumbnails are often tiny
+                # (~90px) and upscale to mush. image_url is filled only by the
+                # quality-gated og:image enrichment (brief / recent / on-open).
+                None,
                 item.language,
                 item.raw_guid,
                 url_hash,