On This Day: serve sharp images (originalimage, not the 330px thumbnail)

The Wikimedia feed's thumbnail is 330px, which upscales blurry in our hero. Use
originalimage.source instead — it's reliably sharp. (Can't just request a bigger
thumbnail width: for very large source images Wikimedia only serves pre-generated
bucket sizes and 400s on arbitrary widths — e.g. 500px ok, 800/1024px fail.)

- onthisday._best_image() prefers originalimage, falls back to the thumbnail.
- scripts/otd_image_upsize_backfill.py re-fetches each stored MM-DD and upgrades
  image_url in onthisday_pool + daily_onthisday in place (ran on host: pool + 6
  daily rows now sharp; today's hero verified 200). Only the /onthisday hero
  loads this image (home card is text-only), so larger files are a single-page,
  one-time load.
- test_best_image locks the prefer-original/fallback behavior.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

goodnews/onthisday.py

@@ -31,6 +31,18 @@ _NEG = (
 )
 
 
+# Wikimedia's feed hands us a 330px `thumbnail`, which upscales (blurry) in our hero. It also
+# gives `originalimage` — a sharp, full-size URL that's always valid. We can't just request a
+# bigger thumbnail width: for very large source images Wikimedia only serves pre-generated
+# bucket sizes and 400s on arbitrary widths (e.g. 500px ok, 800/1024px fail, 1280px ok). So
+# prefer the originalimage (reliably sharp), falling back to the thumbnail.
+def _best_image(page: dict) -> str | None:
+    """The sharpest reliably-served image URL: originalimage, else the 330px thumbnail."""
+    orig = (page.get("originalimage") or {}).get("source")
+    thumb = (page.get("thumbnail") or {}).get("source")
+    return orig or thumb or None
+
+
 def _fetch_events(md: str) -> list[dict]:
     """All events for a MM-DD from Wikimedia, normalized to our candidate shape."""
     mm, dd = md.split("-")
@@ -46,7 +58,7 @@ def _fetch_events(md: str) -> list[dict]:
             "year": e.get("year"),
             "text": text,
             "summary": (page.get("extract") or "").strip() or None,
-            "image_url": ((page.get("thumbnail") or {}).get("source")) or None,
+            "image_url": _best_image(page),
             "page_url": (((page.get("content_urls") or {}).get("desktop") or {}).get("page")) or None,
         })
     return out