Sources table: Media column (image coverage % + paywall marker)

Per Codex — make the table more decision-ready from data we already have.
Paywall is a domain-level hint, so it's a per-source flag (not a meaningful
rate): show image-coverage % plus a 🔒 marker for subscription domains in one
compact "Media" column (tooltip spells it out). source_health gains a
`paywalled` flag (is_paywalled on homepage/feed); also added to sources.csv.

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

goodnews/api.py

@@ -1046,7 +1046,7 @@ def create_app() -> FastAPI:
             row([
                 "name", "feed_url", "homepage", "status", "visible", "served", "accepted_total",
                 "total_articles", "acceptance_pct", "duplicate_pct", "accepted_dup_pct",
-                "image_coverage_pct", "last_success", "last_error", "retry_after", "review_flag", "review_reason",
+                "image_coverage_pct", "paywalled", "last_success", "last_error", "retry_after", "review_flag", "review_reason",
             ])
             for s in rows:
                 row([
@@ -1054,6 +1054,7 @@ def create_app() -> FastAPI:
                     s.get("status") or "", "yes" if s.get("content_visible") else "no",
                     s["served"], s["accepted_total"], s["total_articles"],
                     s["acceptance_rate"], s["duplicate_rate"], s["accepted_dup_rate"], s["image_coverage"],
+                    "yes" if s.get("paywalled") else "no",
                     s.get("last_success_at") or "", s.get("last_error") or "", s.get("retry_after_at") or "",
                     "yes" if s.get("review_flag") else "no", s.get("review_reason") or "",
                 ])