Daily Art engine: museum-guide blurb (grounded LLM) + extracted palette

- daily_art gains blurb + palette columns (idempotent migration).
- art._palette: Pillow median-cut to ~5 hex colors from the cached image (best-
  effort → [] on any failure). art._blurb: a warm 2-3 sentence "what you're
  looking at" note grounded in the Met catalogue (title/artist/bio/date/medium/
  classification/culture/tags). Prompt leans on context/significance and the
  title+tags for subject — explicitly NOT asserting literal composition (figure
  counts/poses) it can't see, since the model can't view the image. Markdown
  stripped from the output.
- pick_daily generates both (client optional → blurb skipped when absent); cycle
  + art CLI pass an LLM client. /api/art/today exposes blurb + palette.
- Backfilled the last 3 days on host (Veteran / Magnolia Vase / Bierstadt).
- scripts/art_blurb_palette_backfill.py for in-place backfill (no re-pick).

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

tests/test_art.py

@@ -44,6 +44,58 @@ def test_harvest_dedupes_into_pool(conn):
     assert art.harvest_pool(conn)["added"] == 0          # idempotent
 
 
+def test_palette_extracts_hex_colors(tmp_path):
+    from PIL import Image
+    p = tmp_path / "img.png"
+    im = Image.new("RGB", (60, 60), (200, 30, 30))        # mostly red...
+    for x in range(60):
+        for y in range(30):
+            im.putpixel((x, y), (30, 150, 70))            # ...top half green
+    im.save(p)
+    cols = art._palette(p, n=3)
+    assert 1 <= len(cols) <= 3
+    assert all(c.startswith("#") and len(c) == 7 for c in cols)
+
+
+def test_palette_bad_image_is_empty(tmp_path):
+    p = tmp_path / "bad.jpg"
+    p.write_bytes(b"\xff\xd8\xff" + b"x" * 500)            # not a decodable image
+    assert art._palette(p) == []
+
+
+class _FakeClient:
+    def __init__(self, text="A quiet wheat field at dusk."):
+        self.text, self.seen = text, None
+    def chat_text(self, messages):
+        self.seen = messages
+        return self.text
+
+
+def test_blurb_grounds_in_metadata_and_cleans():
+    c = _FakeClient("  A returning soldier in a golden field.  \n")
+    out = art._blurb(c, {"title": "The Veteran", "artistDisplayName": "Homer",
+                         "medium": "Oil on canvas", "tags": [{"term": "wheat"}, {"term": "scythe"}]})
+    assert out == "A returning soldier in a golden field."
+    user = c.seen[-1]["content"]
+    assert "Homer" in user and "Oil on canvas" in user and "wheat" in user   # catalogue facts fed in
+
+
+def test_blurb_none_on_error_or_empty():
+    class Bad:
+        def chat_text(self, m): raise RuntimeError("down")
+    assert art._blurb(Bad(), {"title": "X"}) is None
+    assert art._blurb(_FakeClient("   "), {"title": "X"}) is None
+
+
+def test_pick_stores_blurb_and_palette(conn):
+    art.harvest_pool(conn)
+    a = art.pick_daily(conn, art_date="2026-06-21", client=_FakeClient("A quiet masterwork."))
+    assert a["blurb"] == "A quiet masterwork."
+    assert a["palette"] == "[]"        # fixture image isn't decodable → empty palette, stored as JSON
+    b = art.pick_daily(conn, art_date="2026-06-22")    # no client → no blurb, pick still succeeds
+    assert b["blurb"] is None
+
+
 def test_pick_caches_image_metadata_and_marks_shown(conn):
     art.harvest_pool(conn)
     a = art.pick_daily(conn, art_date="2026-06-21")

tests/test_art_api.py

@@ -23,6 +23,8 @@ def client(tmp_path, monkeypatch):
         "VALUES ('2026-06-21','met',10154,'Lander''s Peak','Bierstadt','1863','Oil','Paintings',"
         "'Gift','https://met/10154','10154.jpg','https://met/full.jpg',1)"
     )
+    c.execute("UPDATE daily_art SET blurb=?, palette=? WHERE object_id=10154",
+              ("A luminous western vista.", '["#7fb4cf", "#c79a3c"]'))
     c.commit(); c.close()
     cache.mkdir(parents=True, exist_ok=True)
     (cache / "10154.jpg").write_bytes(b"\xff\xd8\xff" + b"x" * 5000)   # web-large display copy
@@ -43,3 +45,5 @@ def test_today_exposes_full_res_url(client):
     assert a["image_url"] == "/api/art/image/10154"
     assert a["image_url_large"] == "/api/art/image/10154?size=full"
     assert a["license"] == "Public Domain (CC0)" and a["museum"] == "The Met"
+    assert a["blurb"] == "A luminous western vista."
+    assert a["palette"] == ["#7fb4cf", "#c79a3c"]        # parsed from stored JSON