Daily Art backend: curated Met pool, daily cached pick, /api/art (prototype)

The engine for the /art room (design-independent; deploy held for Codex review).

- goodnews/art.py: harvest a curated pool of public-domain HIGHLIGHT artworks from the
  Met (isHighlight+isPublicDomain+hasImages -> masterworks, never potsherds; CC0). Daily
  deterministic pick from the least-recently-shown (no soon-repeats, same for everyone),
  fetch metadata + download the image to OUR cache (data/art_cache) so the homepage never
  waits on or hotlinks the museum. Bulletproof: bad object/image falls through candidates;
  a failed day keeps the last piece (room never empty). Injectable HTTP for tests.
- Schema: art_pool + daily_art. /api/art/today (edge-cacheable) + /api/art/image/{id}
  (served from cache, immutable). CLI `art [--harvest] [--force]` + a non-fatal cycle step.
- Tests (5, mocked HTTP) + verified live against the Met: harvested 1641 works,
  picked/cached "Repose" by John White Alexander. 371 tests green.

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

goodnews/db.py

@@ -245,6 +245,32 @@ CREATE INDEX IF NOT EXISTS idx_article_places_article ON article_places(article_
 CREATE INDEX IF NOT EXISTS idx_article_places_country ON article_places(country_code);
 CREATE INDEX IF NOT EXISTS idx_article_geo_breadth ON article_geo(breadth);
 
+-- Daily Art (the /art room). art_pool = a curated set of public-domain, highlighted
+-- museum object IDs (so the daily pick never hits a potsherd). daily_art = one cached
+-- piece per day (metadata + a locally-cached image), so the homepage never waits on or
+-- hotlinks the museum. shown_at lets us avoid repeating a piece too soon.
+CREATE TABLE IF NOT EXISTS art_pool (
+    object_id INTEGER NOT NULL,
+    source TEXT NOT NULL DEFAULT 'met',
+    shown_at TEXT,
+    added_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (source, object_id)
+);
+CREATE TABLE IF NOT EXISTS daily_art (
+    art_date TEXT PRIMARY KEY,
+    source TEXT NOT NULL DEFAULT 'met',
+    object_id INTEGER NOT NULL,
+    title TEXT,
+    artist TEXT,
+    date_text TEXT,
+    medium TEXT,
+    department TEXT,
+    credit TEXT,
+    source_url TEXT,
+    image_file TEXT,
+    created_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
 -- Privacy-respecting, first-party analytics. NO IP / user-agent / referrer / raw
 -- URL. visitor_hash is a hash of a random localStorage token (never email/IP).
 -- The UNIQUE key dedups to one row per (kind, article, visitor, day) — that both