feat(server): wire up comment Jetstream consumer
Initialize comment repository and Jetstream consumer at server startup.
Consumer runs in background goroutine, indexing comment events from
atProto firehose to PostgreSQL AppView.
Consumer lifecycle:
- Start on server init
- Graceful shutdown on SIGINT/SIGTERM
- Automatic reconnection on connection loss
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Co-Authored-By: Claude <noreply@anthropic.com>