feat(plex): apply the sidebar filters to the Discover rails too

Discover now behaves like the Browse grid — the active metadata filters (genre /
rating / year / collection / studio / …) narrow every rail: Continue watching,
Recently added, Surprise me, the genre tiles, and Because you watched. Same
LibraryFilters / meta_filter_conds the grid uses, applied to each rail's movie +
show query (movie duration included); a rail may come back empty, which the client
already hides. Genre-language matches the grid.

Because Discover honours filters itself, adding a filter no longer force-flips the
Discover landing to the grid (only a header SEARCH does, since Discover has no
search rail) — so you stay on Discover with narrowed rails.

Frontend threads `filters` into api.plexDiscover/plexSurprise + their query keys
(plexSurprise is variadic now) and the watched-toggle setQueryData; PlexDiscover
reads them from usePlex. Tests: two discover filter-narrowing cases; PlexDiscover
dom test updated for the provider default + 3-arg call.

backend/tests/test_plex_discover.py

@@ -200,3 +200,27 @@ def test_discover_scope_movie_excludes_shows(db, monkeypatch):
     assert all(c["type"] == "movie" for c in d["recently_added"])
     # The show drops out of continue watching under the movie scope.
     assert _ids(d["continue_watching"]) == ["m3"]
+
+
+def test_filters_narrow_every_discover_rail(db, monkeypatch):
+    # The sidebar filters constrain the Discover rails just like the Browse grid (ask 3). A genre=Drama
+    # filter keeps only the Drama titles across every rail; the genre-less show and the Comedy rec drop.
+    user = _seed(db)
+    d = _get(db, user, monkeypatch, "/api/plex/discover?scope=both&genres=Drama").json()
+    # Recently-added: m1 + m3 (Drama) survive; Comedy m2 and the genre-less show sh1 are filtered out.
+    assert set(_ids(d["recently_added"])) == {"m1", "m3"}
+    # Continue-watching: only the in-progress Drama movie remains (the show has no genres → excluded).
+    assert _ids(d["continue_watching"]) == ["m3"]
+    # Genre tiles reflect the filtered set — only Drama, count 2 (m1 + m3).
+    assert {g["value"]: g["count"] for g in d["genres"]} == {"Drama": 2}
+    # Because-you-watched: the anchor m1 is watched, but its only rec (m2) is Comedy → filtered → no rail.
+    assert d["because_you_watched"] is None
+
+
+def test_filters_narrow_surprise(db, monkeypatch):
+    user = _seed(db)
+    items = _get(
+        db, user, monkeypatch, "/api/plex/surprise?scope=movie&genres=Comedy"
+    ).json()["items"]
+    # Only the one Comedy movie qualifies under the filter.
+    assert _ids(items) == ["m2"]