Backend: Feature Flag Hygiene

Feature flags in backend should be intentional and maintainable.

Scope:
- Enforce when adding or changing backend feature flags.

Rules:
1) New flags should include clear ownership/context in code or PR description.
2) Default behavior should be explicit and safe.
3) Add a cleanup intent (ticket, comment, or follow-up note) for temporary/experimental flags.

Review behavior:
- Return NOT APPROVED when a new flag is introduced with ambiguous default behavior or no clear ownership/cleanup intent.
- Do not block simple flag removals.