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_person_bio.py

@@ -168,17 +168,14 @@ def test_enrich_person_noop_without_tmdb_id(db, monkeypatch):
     assert plex_tmdb.enrich_person(db, person) is False
 
 
-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 test_person_detail_schedules_enrich_off_request_path(db, monkeypatch):
+def test_person_detail_schedules_enrich_off_request_path(db, monkeypatch, override_dep):
     # The bio fetch runs OFF the request path (a background task after the response), so the serve never
     # makes a live TMDB call. An un-enriched person (tmdb_person_id set + tmdb_enriched_at None) schedules
     # the enrich and the page still renders immediately with whatever's in the row (bio still null).
@@ -196,7 +193,7 @@ def test_person_detail_schedules_enrich_off_request_path(db, monkeypatch):
     monkeypatch.setattr(people, "_enrich_person_bg", lambda pid: scheduled.append(pid))
     people._enrich_inflight.clear()  # ensure the per-person cooldown guard doesn't suppress this open
 
-    resp = _get(db, user, monkeypatch, "/api/plex/person/Lazy P")
+    resp = _get(db, user, monkeypatch, override_dep, "/api/plex/person/Lazy P")
     assert resp.status_code == 200, resp.text
     assert resp.json()["biography"] is None  # not enriched yet — the bio lands by the next open
     assert len(scheduled) == 1  # the enrich was scheduled off the request path
@@ -260,7 +257,7 @@ def test_should_schedule_enrich_dedups_within_cooldown():
     people._enrich_inflight.clear()
 
 
-def test_person_detail_no_enrich_when_already_enriched(db, monkeypatch):
+def test_person_detail_no_enrich_when_already_enriched(db, monkeypatch, override_dep):
     # An already-enriched person (tmdb_enriched_at set) must NOT schedule another background fetch.
     from datetime import datetime, timezone
 
@@ -284,13 +281,13 @@ def test_person_detail_no_enrich_when_already_enriched(db, monkeypatch):
     scheduled: list[int] = []
     monkeypatch.setattr(people, "_enrich_person_bg", lambda pid: scheduled.append(pid))
 
-    resp = _get(db, user, monkeypatch, "/api/plex/person/Done P")
+    resp = _get(db, user, monkeypatch, override_dep, "/api/plex/person/Done P")
     assert resp.status_code == 200, resp.text
     assert resp.json()["biography"] == "Bio."
     assert scheduled == []  # already enriched → no background work
 
 
-def test_person_detail_serves_bio_and_resolves_known_for(db, monkeypatch):
+def test_person_detail_serves_bio_and_resolves_known_for(db, monkeypatch, override_dep):
     lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
     db.add(lib)
     db.flush()
@@ -327,7 +324,7 @@ def test_person_detail_serves_bio_and_resolves_known_for(db, monkeypatch):
     db.add(user)
     db.commit()
 
-    resp = _get(db, user, monkeypatch, "/api/plex/person/Actor X")
+    resp = _get(db, user, monkeypatch, override_dep, "/api/plex/person/Actor X")
     assert resp.status_code == 200, resp.text
     j = resp.json()
     assert j["biography"] == "Bio here."