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/db.py

@@ -817,6 +817,11 @@ def init_db():
         from app.migrations import apply_migrations
         apply_migrations(conn)
 
+    # Webhook subscriptions (v2.1.0) — created here (idempotent) so the full
+    # schema exists without depending on the FastAPI lifespan startup.
+    from app.services.webhook_outbound import init_webhook_tables
+    init_webhook_tables()
+
 
 @contextmanager
 def get_conn():