test(fixtures): make the TestClient fixtures isolated and composable (S2 review)

Address the /code-review findings on the S2 test-net:
- as_user no longer sets a global `lambda: user` override that two users in one
  test would collide on; the shared current_user override now resolves the caller
  from an x-test-user header, and each returned client tags its own requests, so
  as_user(a)/as_user(b) are independent (#1).
- override_dep records the prior value per dependency key and restores it on
  teardown instead of a blanket dependency_overrides.clear(), so fixtures compose
  without clobbering each other (#5).
- a _configured fixture bakes the post-setup state into client/as_user, and the
  per-test inline monkeypatch / autouse duplicates are removed (#4).
- the feed keyset walk asserts all seeded rows surfaced before comparing, so a
  seeding regression can't make the sorts pass vacuously (#3).

backend/tests/test_feed_keyset.py

@@ -15,7 +15,6 @@ from datetime import datetime, timezone
 
 import pytest
 
-import app.state as state
 from app.models import Channel, Subscription, User, Video
 
 # Every sort feedquery.sort_keys defines except "relevance" (which needs a search query to rank).
@@ -74,8 +73,7 @@ def _page(client, sort, cursor):
     return resp.json()
 
 
-def test_feed_surfaces_the_whole_seed(db, as_user, monkeypatch):
-    monkeypatch.setattr(state, "_configured_cache", True)  # the request gate 503s an unconfigured app
+def test_feed_surfaces_the_whole_seed(db, as_user):
     user, ids = _seed(db)
     body = as_user(user).get("/api/feed?limit=50").json()
     assert {it["id"] for it in body["items"]} == set(ids)
@@ -83,13 +81,15 @@ def test_feed_surfaces_the_whole_seed(db, as_user, monkeypatch):
 
 
 @pytest.mark.parametrize("sort", _SORTS)
-def test_cursor_walk_matches_the_unpaged_order(db, as_user, monkeypatch, sort):
-    monkeypatch.setattr(state, "_configured_cache", True)
-    user, _ = _seed(db)
+def test_cursor_walk_matches_the_unpaged_order(db, as_user, sort):
+    user, ids = _seed(db)
     client = as_user(user)
 
     # Ground truth: the full order in one query (no cursor).
     full = [it["id"] for it in client.get(f"/api/feed?sort={sort}&limit=50").json()["items"]]
+    # Guard against a vacuous pass: if seeding ever stops surfacing the videos, `walked == full` would
+    # be [] == [] for every sort and never exercise the cursor. All 7 rows must be present.
+    assert len(full) == len(ids), f"{sort}: seed did not surface all rows — the walk would be vacuous"
 
     # Walk it two at a time via the cursor and reassemble.
     walked: list[str] = []