Segment membership: export

[khvn26]

Scope

• Async job that iterates the segment's bitmap end-to-end, resolves ords → identifiers, streams CSV / JSONL to S3, returns a signed download URL.
• Audit-logged with rate limiting.
• Streamed, not buffered.

Acceptance criteria

• Download URL is valid for a configurable TTL.
• A 1M-member segment exports without buffering more than a configurable in-memory ceiling (concrete value TBD by review).
• Audit log captures who, when, segment id, env id, row count.