Add structured JSON logging for production deployments

[haydenk]

tracing-subscriber is currently configured for human-readable (pretty-print) output only. Production deployments feeding logs to aggregators (Loki, Datadog, Grafana Cloud, etc.) need structured JSON output.

Proposed change:

• Add a LOG_FORMAT environment variable accepting pretty (default) or json
• When json, use tracing_subscriber::fmt().json() formatter
• Apply consistently across all three binaries (downloader, etl, api)

Dependencies to add: tracing-subscriber already has the json feature — just needs to be enabled.