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/agent.py

@@ -178,7 +178,7 @@ async def create_agent(request: Request):
             )
             conn.commit()
         except Exception as exc:  # noqa: BLE001
-            raise HTTPException(status_code=409, detail=f"Impossible de créer l'agent: {exc}")
+            raise HTTPException(status_code=409, detail=f"Impossible de créer l'agent: {exc}") from exc
     return {"id": cur.lastrowid, "name": name, "status": "created"}
 
 
@@ -466,9 +466,9 @@ async def undo(request: Request, action_id: int):
     try:
         undo_action(action_id)
     except ValueError as exc:
-        raise HTTPException(status_code=400, detail=str(exc))
+        raise HTTPException(status_code=400, detail=str(exc)) from exc
     except Exception as exc:  # noqa: BLE001
-        raise HTTPException(status_code=500, detail=f"Rollback échoué: {exc}")
+        raise HTTPException(status_code=500, detail=f"Rollback échoué: {exc}") from exc
     return {"id": action_id, "status": "reverted"}
 
 
@@ -501,7 +501,7 @@ async def create_skill(request: Request):
             )
             conn.commit()
         except Exception as exc:  # noqa: BLE001
-            raise HTTPException(status_code=409, detail=f"Skill existe déjà: {exc}")
+            raise HTTPException(status_code=409, detail=f"Skill existe déjà: {exc}") from exc
     return {"id": cur.lastrowid, "name": name, "status": "created"}