Streamline segment mutations in the frontend

[khvn26]

Move the normal segment edit flow to full-replace semantics. Drop the delete: true convention both from frontend and backend.

Acceptance criteria

• Removing a rule or condition in the editor omits it from the request body; no entry carries delete: true.
• SegmentRule and SegmentCondition types no longer carry id.
• Editing a segment work against the id-free API.
• API does not expect delete: true anymore.
• The change request segment diff renders unchanged.

[matthewelwell]

Both sides ship in one PR since the frontend and API live in the same repo and deploy together.

@khvn26 this isn't true for SaaS or a lot of self-hosted deployments. We'll have to have either the FE or the API be able to support both paths.

[khvn26]

@matthewelwell Not a concern; the backend is backwards-compatible up to this point, and we guarantee minor-version compat.