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_canvas.py

@@ -1,15 +1,8 @@
 import uuid
 
-import pytest
 from httpx import AsyncClient
 
 
-@pytest.fixture
-async def headers(client: AsyncClient):
-    res = await client.post("/api/v1/auth/login", json={"username": "admin", "password": "admin"})
-    return {"Authorization": f"Bearer {res.json()['access_token']}"}
-
-
 def node_payload(**kwargs):
     return {"id": str(uuid.uuid4()), "type": "server", "label": "N", "status": "unknown", "pos_x": 0, "pos_y": 0, **kwargs}