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>

tests/test_dashboard.py

@@ -127,3 +127,21 @@ def test_attention_ignores_rate_limit_on_paused_or_retired():
     ]
     # neither should nag about a rate-limit rest
     assert queries._attention(content, sources, 0, now=now) == []
+
+
+def test_source_health_paywall_and_image_coverage(tmp_path):
+    import sqlite3
+    from goodnews.db import connect, init_db
+    from goodnews import queries
+    c = connect(str(tmp_path / "t.db")); init_db(c)
+    # a paywalled-domain source and a free one, each with a served article
+    c.execute("INSERT INTO sources (id,name,feed_url,homepage_url,active) VALUES (1,'NS','http://x/f','https://www.nature.com',1)")
+    c.execute("INSERT INTO sources (id,name,feed_url,homepage_url,active) VALUES (2,'Free','http://y/f','https://goodsite.org',1)")
+    for aid, sid, img in [(1, 1, 'http://i/1.jpg'), (2, 2, None)]:
+        c.execute("INSERT INTO articles (id,source_id,canonical_url,title,url_hash,image_url) VALUES (?,?,?,?,?,?)",
+                  (aid, sid, f'http://u/{aid}', f't{aid}', f'h{aid}', img))
+        c.execute("INSERT INTO article_scores (article_id,accepted) VALUES (?,1)", (aid,))
+    c.commit()
+    sh = {s["id"]: s for s in queries.source_health(c)}
+    assert sh[1]["paywalled"] is True and sh[2]["paywalled"] is False
+    assert sh[1]["image_coverage"] == 100 and sh[2]["image_coverage"] == 0