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/automations.py

@@ -53,7 +53,7 @@ def _validate_payload(body: dict) -> None:
             else:
                 json.dumps(val)
         except (TypeError, json.JSONDecodeError):
-            raise HTTPException(status_code=400, detail=f"{key} must be valid JSON")
+            raise HTTPException(status_code=400, detail=f"{key} must be valid JSON") from None
 
 
 @router.get("/workspace/automations")