feat(plex): Surprise-me shows only unseen titles, capped 3 movies + 3 shows (C)

The Surprise rail drew a random 20+20 sample including already-watched titles. Its point is
to surface something to WATCH, so exclude seen titles (a watched movie, a fully-watched show
— a partially-watched or never-started show still counts as unseen) and cap at 3 movies + 3
shows (6 total), interleaved.

Verified live: /surprise returns 6 items, 3+3, none watched.

Gate: backend ruff clean, 12 discover tests (+ a watched-exclusion/cap assertion).

backend/tests/test_plex_discover.py

@@ -204,6 +204,18 @@ def test_surprise_scope_and_bound(db, monkeypatch):
     assert all(c["type"] == "movie" for c in items)  # scope=movie excludes shows
 
 
+def test_surprise_excludes_watched_and_caps_per_kind(db, monkeypatch):
+    # Surprise surfaces only UNSEEN titles: the watched movie (m1) is out; the unwatched (m2), the
+    # in-progress movie (m3) and the partially-watched show (sh1) are eligible. Capped at 3 per kind.
+    user = _seed(db)
+    items = _get(db, user, monkeypatch, "/api/plex/surprise?scope=both").json()["items"]
+    ids = _ids(items)
+    assert "m1" not in ids  # watched → excluded
+    assert set(ids) <= {"m2", "m3", "sh1"}  # only unseen titles
+    assert len([c for c in items if c["type"] == "movie"]) <= 3
+    assert len([c for c in items if c["type"] == "show"]) <= 3
+
+
 def test_discover_scope_movie_excludes_shows(db, monkeypatch):
     user = _seed(db)
     d = _get(db, user, monkeypatch, "/api/plex/discover?scope=movie").json()