test(be): every dependency override goes through override_dep

22 test files set `app.dependency_overrides[...]` by hand and tore down with a blanket `.clear()`
in a `finally`, which wipes every override rather than the test's own and stops fixtures composing.
They now take the `override_dep` fixture (helpers get it passed in), whose teardown restores each
key's prior value; no clear() carried meaning between two phases of a test. A convention test fails
any test file that writes `dependency_overrides` directly.

backend/tests/test_plex_field_scoped_search.py

@@ -100,14 +100,11 @@ def _seed(db):
     return {"user": user}
 
 
-def _get(db, user, monkeypatch, path):
+def _get(db, user, monkeypatch, override_dep, path):
     monkeypatch.setattr(state, "_configured_cache", True)
-    app.dependency_overrides[get_db] = lambda: db
-    app.dependency_overrides[current_user] = lambda: user
-    try:
-        return TestClient(app).get(path)
-    finally:
-        app.dependency_overrides.clear()
+    override_dep(get_db, lambda: db)
+    override_dep(current_user, lambda: user)
+    return TestClient(app).get(path)
 
 
 def _ids(body):
@@ -115,55 +112,75 @@ def _ids(body):
 
 
 class TestFieldScopedSearch:
-    def test_everywhere_matches_every_facet(self, db, monkeypatch):
+    def test_everywhere_matches_every_facet(self, db, monkeypatch, override_dep):
         s = _seed(db)
-        body = _get(db, s["user"], monkeypatch, "/api/plex/library?q=zephyr&field=all").json()
+        body = _get(
+            db, s["user"], monkeypatch, override_dep, "/api/plex/library?q=zephyr&field=all"
+        ).json()
         # title (mt), cast (mp), summary (ms) all land in the item grid; the episode is grouped out
         assert _ids(body) == ["mp", "ms", "mt"]
         assert [e["title"] for e in body["episodes"]] == ["Zephyr Episode"]
 
-    def test_field_all_is_identical_to_default(self, db, monkeypatch):
+    def test_field_all_is_identical_to_default(self, db, monkeypatch, override_dep):
         s = _seed(db)
-        default = _get(db, s["user"], monkeypatch, "/api/plex/library?q=zephyr").json()
-        scoped = _get(db, s["user"], monkeypatch, "/api/plex/library?q=zephyr&field=all").json()
+        default = _get(
+            db, s["user"], monkeypatch, override_dep, "/api/plex/library?q=zephyr"
+        ).json()
+        scoped = _get(
+            db, s["user"], monkeypatch, override_dep, "/api/plex/library?q=zephyr&field=all"
+        ).json()
         assert _ids(default) == _ids(scoped) == ["mp", "ms", "mt"]
         assert default["total"] == scoped["total"]
         assert [e["id"] for e in default["episodes"]] == [e["id"] for e in scoped["episodes"]]
 
-    def test_title_scope_narrows_to_title_match(self, db, monkeypatch):
+    def test_title_scope_narrows_to_title_match(self, db, monkeypatch, override_dep):
         s = _seed(db)
-        body = _get(db, s["user"], monkeypatch, "/api/plex/library?q=zephyr&field=title").json()
+        body = _get(
+            db, s["user"], monkeypatch, override_dep, "/api/plex/library?q=zephyr&field=title"
+        ).json()
         assert _ids(body) == ["mt"]
         assert body["total"] == 1
         assert body["episodes"] == []  # non-episode field scopes suppress the episode section
 
-    def test_people_scope_narrows_to_cast_match(self, db, monkeypatch):
+    def test_people_scope_narrows_to_cast_match(self, db, monkeypatch, override_dep):
         s = _seed(db)
-        body = _get(db, s["user"], monkeypatch, "/api/plex/library?q=zephyr&field=people").json()
+        body = _get(
+            db, s["user"], monkeypatch, override_dep, "/api/plex/library?q=zephyr&field=people"
+        ).json()
         assert _ids(body) == ["mp"]
 
-    def test_summary_scope_narrows_to_description_match(self, db, monkeypatch):
+    def test_summary_scope_narrows_to_description_match(self, db, monkeypatch, override_dep):
         s = _seed(db)
-        body = _get(db, s["user"], monkeypatch, "/api/plex/library?q=zephyr&field=summary").json()
+        body = _get(
+            db, s["user"], monkeypatch, override_dep, "/api/plex/library?q=zephyr&field=summary"
+        ).json()
         assert _ids(body) == ["ms"]
 
-    def test_episode_scope_empties_grid_and_returns_episode(self, db, monkeypatch):
+    def test_episode_scope_empties_grid_and_returns_episode(self, db, monkeypatch, override_dep):
         s = _seed(db)
-        body = _get(db, s["user"], monkeypatch, "/api/plex/library?q=zephyr&field=episode").json()
+        body = _get(
+            db, s["user"], monkeypatch, override_dep, "/api/plex/library?q=zephyr&field=episode"
+        ).json()
         assert body["items"] == []
         assert body["total"] == 0
         assert [e["title"] for e in body["episodes"]] == ["Zephyr Episode"]
 
-    def test_episode_scope_needs_shows_in_scope(self, db, monkeypatch):
+    def test_episode_scope_needs_shows_in_scope(self, db, monkeypatch, override_dep):
         s = _seed(db)
         body = _get(
-            db, s["user"], monkeypatch, "/api/plex/library?q=zephyr&field=episode&scope=movie"
+            db,
+            s["user"],
+            monkeypatch,
+            override_dep,
+            "/api/plex/library?q=zephyr&field=episode&scope=movie",
         ).json()
         # movie-only scope has no show libraries -> no episodes to surface, and the grid stays empty
         assert body["items"] == []
         assert body["episodes"] == []
 
-    def test_unknown_field_falls_back_to_everywhere(self, db, monkeypatch):
+    def test_unknown_field_falls_back_to_everywhere(self, db, monkeypatch, override_dep):
         s = _seed(db)
-        body = _get(db, s["user"], monkeypatch, "/api/plex/library?q=zephyr&field=bogus").json()
+        body = _get(
+            db, s["user"], monkeypatch, override_dep, "/api/plex/library?q=zephyr&field=bogus"
+        ).json()
         assert _ids(body) == ["mp", "ms", "mt"]