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.

Scope

1. Instead of sending delete: true, the frontend omits removed entries and sends the desired final rules array, as the change-request flow already does via stripIdsAndDeletedFromRulesAndConditions.
2. Drop the optional id from the SegmentRule and SegmentCondition TypeScript types — the frontend never reads it.
3. Remove SegmentSerializer._get_rules_and_conditions_without_deleted.

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; typecheck passes.
• Creating, editing, and cloning a segment work against the id-free API.
• The change-request segment diff renders unchanged.
• set_segment_rules no longer strips a delete flag.

[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.