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/og_fetcher.py

@@ -29,7 +29,7 @@ def _extract_og(html: str) -> dict:
     text = html[:400_000]  # only scan the beginning — that's where <head> lives
     props: dict[str, str] = {}
     for m in _META_RE.finditer(text):
-        groups = m.groups()
+        m.groups()
         content = ""
         prop = ""
         for s in re.findall(r"(?:content|property|name)=[\"']([^\"']*)[\"']", m.group(0)):
@@ -121,4 +121,4 @@ async def fetch_og_metadata(url: str, timeout: float = 6.0) -> dict:
         "image": abs_url(img),
         "site_name": site.strip()[:100],
         "favicon": favicon,
-    }
+    }