feat(event): accept member_id in event ingestion API

[psincraian]

Summary

Update the event ingestion API to accept member fields, as part of Phase 6: Event Ingestion (parent issue #7899).

Context

From Slack discussion with Pieter:

• Events should support member_id / external_member_id to track "who did this" on top of "who pays for this"
• B2C (seat_based disabled): Can auto-map customer to owner member (future enhancement)
• B2B: Optionally pass [external_]member_id for member-specific attribution
• Billing queries remain unchanged (still use [external_]customer_id)

Changes

Schema Changes (server/polar/event/schemas.py)

• Add member_id: UUID | None to EventCreateCustomer schema
• Add external_member_id: str | None to EventCreateExternalCustomer schema

Service Changes (server/polar/event/service.py)

• Update ingest() method to accept and validate member fields
• Store member_id and external_member_id in Event record
• Copy member fields to MeterEvent records in _create_meter_events()

Endpoint Changes (server/polar/event/endpoints.py)

• No changes needed - schemas drive the API contract

Files to Modify

• server/polar/event/schemas.py
• server/polar/event/service.py

Deployment

Safe to deploy - optional parameters maintain backward compatibility.

Parent Issue

Part of #7899

Dependencies

Requires #9224 and #9226 (database migrations) to be completed first