Moves the risk baseline (initial_risk_level) for a batch of addresses to a caller-supplied risk level, without re-screening.

Semantics:

• Each entry carries a risk_level (SEVERE, HIGH, MEDIUM, or LOW) that becomes the member's new baseline.
• No re-screen is performed; the member's latest effective risk level is untouched — this API only moves the baseline.
• A REBASELINE history row records old-to-new baseline and the actor.
• Batch addresses only; at most 1000 per request.
• Per-address outcomes: APPLIED (baseline moved), NO_OP (baseline already at the target), NOT_FOUND (address not a live member of the pool).
• Idempotent: re-running the same request is a no-op.