Per-key secrets-backend config options are not recognised as sensitive under a team-scoped spelling

### What happened `airflow/api_fastapi/core_api/services/public/config.py` treats the synthetic per-key secrets-backend options as sensitive by matching **literal** section names: ```python _PER_KEY_SENSITIVE_PREFIXES: dict[str, str] = { "secrets": "backend_kwarg__", "workers": "secrets_backend_kwarg__", } ``` Both `_is_per_key_sensitive_option` and `_mask_per_key_sensitive_options` key off those exact names, so neither recognises a team-scoped spelling of the same option: - the config-file section `[<team>=secrets]`, and - `AIRFLOW__<TEAM>___SECRETS__BACKEND_KWARG__*`, which is reported under a section named after the *team* rather than after `secrets`. This is the same root cause as apache/airflow#70755 — sensitivity being decided from a section name that a team-scoped override does not use — but a different mechanism, and it was left untouched there. ### Why it is not currently exploitable `_get_custom_secret_backend` is not team-aware, so there is no way to configure a team-scoped `backend_kwarg__*` that the literal-section match could miss. Nothing leaks today. The gap becomes live the moment secrets backends gain team scoping, and at that point it fails open — a team's backend credentials would be returned in full by `GET /config`. ### What needs doing Resolve both spellings back to the base option before deciding sensitivity, as `AirflowConfigParser.is_sensitive_option` now does for registered options. The env-var spelling is the awkward half: it lands under a section named after the team, which is the shape `_names_sensitive_team_env_var` deals with in the shared parser. The synthetic per-key options are built separately and need their own pass over `conf_dict`. ### Acceptance criteria - A team-scoped `backend_kwarg__*` / `secrets_backend_kwarg__*` value is redacted by `GET /config` and `GET /config/section/{section}/option/{option}` when `display_sensitive=False`, under both the `[<team>=secrets]` and the `AIRFLOW__<TEAM>___SECRETS__...` spelling. -…