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/services/notifications.py

@@ -94,7 +94,7 @@ def _do_mentions(conn, handles, actor_id, ntype, title, message,
                  resource_type, resource_id, url, notified):
     placeholders = ",".join("?" * len(handles))
     rows = conn.execute(
-        "SELECT id, login, email FROM users WHERE lower(login) IN (%s)" % placeholders,
+        f"SELECT id, login, email FROM users WHERE lower(login) IN ({placeholders})",
         handles,
     ).fetchall()
     for row in rows: