Daily Art: Codex guardrails (atomic image, attribution/license, blocked lever)

Hardening before it runs further on the cycle:
- DB-lock/network: all HTTP (metadata + image) happens before any write; the write txn
  opens only at the brief INSERT and commits immediately. Images download to a temp file
  then atomic os.replace into cache (a reader never sees a half-written file).
- Site-timezone "daily" already used local_today() (same rhythm as the Brief) — confirmed.
- Attribution from day one: store + return title/artist/date/medium/department/credit/
  source_url/object_id/source + museum name + is_public_domain license marker + the full-
  res source URL (for a richer /art view later). UI can show: Title · Artist · The Met.
- "highlight != always beautiful": added a manual `blocked` flag on art_pool (excluded
  from picks) as the cheap curation lever; a featured override can follow.

Schema migrated (existing art tables get the new columns). 373 tests green.

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

tests/test_art.py

@@ -49,7 +49,17 @@ def test_pick_caches_image_metadata_and_marks_shown(conn):
     a = art.pick_daily(conn, art_date="2026-06-21")
     assert a and a["object_id"] in (1, 3) and a["title"] in ("Sunflowers", "Irises")
     assert a["artist"] == "Van Gogh" and a["image_file"]
+    assert a["is_public_domain"] == 1                               # license marker stored
     assert list(art.cache_dir().glob(f"{a['object_id']}.*"))        # image cached locally
+    assert not list(art.cache_dir().glob("*.tmp"))                  # atomic write left no temp file
+
+
+def test_blocked_pieces_are_never_picked(conn):
+    art.harvest_pool(conn)
+    conn.execute("UPDATE art_pool SET blocked=1 WHERE object_id=1")  # block the good one
+    conn.commit()
+    a = art.pick_daily(conn, art_date="2026-06-21")
+    assert a is None or a["object_id"] != 1                          # never the blocked piece
     shown = conn.execute("SELECT shown_at FROM art_pool WHERE object_id=?", (a["object_id"],)).fetchone()[0]
     assert shown == "2026-06-21"