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.

static/js/app.js

@@ -22,7 +22,7 @@
       xhr.open('POST', '/api/frontend-error', true);
       xhr.setRequestHeader('Content-Type', 'application/json');
       xhr.send(JSON.stringify(error));
-    } catch(e) { /* fail silently */ }
+    } catch { /* fail silently */ }
   }
 
   window.addEventListener('error', function(e) {