feat(plex): episodes, taglines and genres in Hungarian from the HU overlay (E30 S1a)

The HU TV section lists every episode with its Hungarian title and summary, but the
overlay read only movies and shows, so every episode stayed English. It now overlays
the episodes too, and the HU tagline (new tagline_hu column, served on ?lang=hu).

Plex's HU agent returns a mixed genre vocabulary (Action beside Dráma), and a title
with no HU twin kept its English list, so the HU genre list showed Drama and Dráma.
plex/genres.py maps every genre word to one Hungarian word; the overlay writes it
into genres_hu for every row, and a HU genre filter is read through the same map,
so a saved filter still holding Action keeps matching.

backend/tests/test_plex_overlay.py

@@ -10,7 +10,8 @@ from sqlalchemy import func, select
 
 import app.plex.sync as syncmod
 from app import sysconfig
-from app.models import PlexItem, PlexLibrary
+from app.models import PlexItem, PlexLibrary, PlexShow
+from app.plex.genres import hu_genre, hu_genres
 from app.plex.sync import _apply_item, _canonical_guid, _sync_overlay
 from app.plex.query import content_libs
 
@@ -244,3 +245,100 @@ def test_content_libs_excludes_overlay_libraries(db):
     assert en_lib.id in movie_ids
     assert overlay.id not in movie_ids  # the HU overlay is never browsable
     assert show_ids == [en_shows.id]
+
+
+class _KindStub:
+    """An overlay section listing per Plex type (2 = show, 4 = episode), like a HU TV section."""
+
+    def __init__(self, by_type: dict[int, list[dict]]):
+        self._by = by_type
+
+    def section_items(self, key, item_type, start, size):
+        items = self._by.get(item_type, [])
+        return items[start : start + size], len(items)
+
+
+def test_a_show_overlay_writes_hungarian_episode_titles_and_summaries(db):
+    """E30: the HU TV section lists every episode in Hungarian; they were never read, so a Hungarian
+    reader saw every episode title and summary in English."""
+    en_lib = PlexLibrary(plex_key="1", title="TV Shows", kind="show")
+    hu_lib = PlexLibrary(plex_key="9", title="Sorozatok", kind="show", metadata_lang="hu")
+    db.add_all([en_lib, hu_lib])
+    db.flush()
+    show = PlexShow(rating_key="s1", library_id=en_lib.id, title="3 Body Problem", guid="tmdb:10")
+    db.add(show)
+    db.flush()
+    ep = PlexItem(
+        rating_key="e1",
+        library_id=en_lib.id,
+        kind="episode",
+        show_id=show.id,
+        title="Countdown",
+        summary="EN.",
+        guid="tmdb:501",
+    )
+    db.add(ep)
+    db.commit()
+    stub = _KindStub(
+        {
+            syncmod._TYPE_SHOW: [_hu_movie("hs", "A 3-test-probléma", "HU.", [], "tmdb://10")],
+            syncmod._TYPE_EPISODE: [
+                _hu_movie("he", "Visszaszámlálás", "Magyar epizód.", [], "tmdb://501")
+            ],
+        }
+    )
+    _sync_overlay(db, stub, hu_lib, en_lib.id, {})
+    db.commit()
+    db.refresh(ep)
+    db.refresh(show)
+    assert (ep.title_hu, ep.summary_hu) == ("Visszaszámlálás", "Magyar epizód.")
+    assert show.title_hu == "A 3-test-probléma"
+
+
+def test_the_overlay_gives_every_row_one_hungarian_word_per_genre(db):
+    """E30: Plex's HU agent mixes "Action" with "Dráma", and a title with no HU twin kept only its
+    English list — so the HU genre list offered "Drama" beside "Dráma"."""
+    en_lib = PlexLibrary(plex_key="4", title="Movies", kind="movie")
+    hu_lib = PlexLibrary(plex_key="9", title="Filmek", kind="movie", metadata_lang="hu")
+    db.add_all([en_lib, hu_lib])
+    db.flush()
+    twin = PlexItem(
+        rating_key="m1",
+        library_id=en_lib.id,
+        kind="movie",
+        title="Heat",
+        guid="tmdb:1",
+        genres=["Action", "Crime"],
+    )
+    lone = PlexItem(
+        rating_key="m2",
+        library_id=en_lib.id,
+        kind="movie",
+        title="Solo",
+        guid="tmdb:2",
+        genres=["Drama", "Science Fiction"],
+    )
+    db.add_all([twin, lone])
+    db.commit()
+    meta = _hu_movie(
+        "h1", "Szemtől szemben", "HU.", ["Action", "Bűnügyi", "Drama", "Dráma"], "tmdb://1"
+    )
+    meta["tagline"] = "Magyar mottó."
+    _sync_overlay(db, _StubPlex([meta]), hu_lib, en_lib.id, {})
+    db.commit()
+    db.refresh(twin)
+    db.refresh(lone)
+    assert twin.genres_hu == ["Akció", "Bűnügyi", "Dráma"]
+    assert twin.tagline_hu == "Magyar mottó."
+    assert lone.genres_hu == ["Dráma", "Sci-Fi"]  # no twin: its own English list, in Hungarian
+    assert lone.tagline_hu is None
+
+
+def test_the_genre_map_is_idempotent_and_keeps_an_unknown_word():
+    assert hu_genre("science fiction") == "Sci-Fi"  # case-insensitive
+    assert hu_genre("Dráma") == "Dráma"
+    assert hu_genre("Space Opera") == "Space Opera"  # unknown: shown as Plex gave it, never dropped
+    once = hu_genres(["Drama", "Action", "Dráma"])
+    assert once == ["Dráma", "Akció"]
+    assert hu_genres(once) == once
+    assert hu_genres(None) is None and hu_genres([]) is None