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/export.py

@@ -243,7 +243,8 @@ def _parse_table_at(lines: list[str], i: int, n: int):
         rows.append(cells)
         j += 1
     width = max(len(r) for r in rows)
-    pad = lambda r: r + [""] * (width - len(r))
+    def pad(r):
+        return r + [""] * (width - len(r))
     align = (align + ["left"] * width)[:width]
     return (
         {
@@ -657,7 +658,7 @@ def blocks_to_html(blocks: list) -> str:
             url = b.get("src") or ""
             emb = (b.get("embed_type") or "")
             if emb in ("inline_dbs", "collection"):
-                parts.append(f'<div class="embed-note">[Embedded content]</div>')
+                parts.append('<div class="embed-note">[Embedded content]</div>')
             elif emb == "download":
                 parts.append(f'<a href="{_text(url)}" download>⬇ {_text(b.get("file_name") or "Download")}</a>')
             elif emb == "pdf" and url: