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.

tests/test_search_migrations.py

@@ -20,6 +20,11 @@ def client():
     os.environ["APP_SECRET_KEY"] = "test-secret-for-tests"
     os.environ["RATE_LIMIT_ENABLED"] = "false"
 
+    # Point the process-wide settings singleton at OUR temp DB (xdist-safe).
+    from app.config import settings
+    settings.database_url = f"sqlite:///{db_path}"
+    settings.rate_limit_enabled = False
+
     from app.db import init_db
     from app.main import app
     init_db()