Spike: Postgres → ClickHouse CDC

### Goal

To run the membership feature self-hosted with ClickHouse as the warehouse, we need a Postgres → ClickHouse CDC pipeline. Compare:

- Debezium + Kafka Connect — battle-tested, heavy.
- PeerDB — Postgres-native, lighter operational footprint.
- Materialize / Flink — overkill but worth quantifying.
- Custom logical-replication consumer — minimal deps, more code.

### Output

- A user-facing docs PR including a written tradeoff comparison and a rough cost/maintenance budget for self-hosted customers.
- An issue or epic scoping work for the chosen implementation.