test: unify auth headers into a single conftest fixture

Replace 9 duplicated per-file 'headers' fixtures and the awkward
'auth_headers' coroutine factory with one canonical 'headers' fixture
in conftest.py. Migrate liveview/media tests off the await-based
factory. No behavior change; 627 passed.

ha-relevant: no

backend/tests/test_scan.py

@@ -12,13 +12,6 @@ from app.db.models import Design, Node, PendingDevice, ScanRun
 from app.services.scanner import _cancelled_runs, request_cancel, run_scan
 
 
-@pytest.fixture
-async def headers(client: AsyncClient):
-    res = await client.post("/api/v1/auth/login", json={"username": "admin", "password": "admin"})
-    token = res.json()["access_token"]
-    return {"Authorization": f"Bearer {token}"}
-
-
 @pytest.fixture
 async def pending_device(db_session):
     import uuid