Build a Realtime Change Feed (Practical)

Question

## Problem

Implement a change feed: clients subscribe to a table/filter and receive inserts/updates/deletes (Supabase Realtime over Postgres logical replication).

## Example
```
subscribe(table, filter) -> stream of change events
```

## Constraints
- Fan out only matching changes

## Approach
Tail the WAL/replication stream; match against subscriptions; fan out. Discuss back-pressure.