feat(v5.2.0): finalize Infrastructure & Polish (tests isolation, xdist, lint, CI)

tests/conftest.py: mutate the settings singleton (instead of rebinding) so DB + backup dir are isolated per test -> pytest-xdist safe.
Real backup tests (snapshot/prune/admin API) and OAuth mock tests (Gitea/GitHub/link) replace the previous skips.
init_db() now also creates webhook_subscriptions (full schema without the FastAPI lifespan).
ruff check is clean; .eslintrc.json migrated to eslint.config.mjs (flat config).
CI: lint job (ruff + eslint), parallel tests (-n auto), run on every branch push.
VERSION 5.11.1.

app/routers/notifications.py

@@ -107,7 +107,7 @@ async def set_prefs(request: Request):
 @router.get("/users/search")
 async def search_users(request: Request, q: str = ""):
     """User autocomplete for @mentions."""
-    user = _current_user(request)
+    _current_user(request)
     q = (q or "").strip()
     with get_conn() as conn:
         if q: