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/workspace.py

@@ -512,13 +512,13 @@ async def assign_page_to_sprint(request: Request, collection_id: int, sid: int):
             raise HTTPException(404, "Page not found")
 
         try:
-            cur = conn.execute(
+            conn.execute(
                 "INSERT INTO sprint_pages (sprint_id, page_id, status_at_start, velocity_points) VALUES (?,?,?,?)",
                 (sid, page_id, status_at_start, velocity_points),
             )
             conn.commit()
         except sqlite3.IntegrityError:
-            raise HTTPException(409, "Page already assigned to this sprint")
+            raise HTTPException(409, "Page already assigned to this sprint") from None
     return {"sprint_id": sid, "page_id": page_id, "status": "assigned"}