[P1] Reduce per-block log volume and add --log-format=json #43

Priority: P1 (cost) · Effort: S

Problem

Each block produces at least 3 Info log lines ("processing block", "found matching commitment", "… verified on Celestia"), written through zerolog.ConsoleWriter, which is slow and not machine-parseable. At 10 blocks/s that's about 2.6M lines a day. The drift, balance and jsonrpc exporters also log at Info on every successful poll.

Proposed fix

• Move per-block and per-poll success logs to Debug; keep Warn and Error.
• Add a --log-format=json|console flag. Default console so existing behaviour doesn't change. Document it in the README.

Pros / Cons

• ✅ Large reduction in log storage cost; JSON output can be ingested by Loki and ELK.
• ⚠️ Less output by default. The self-observability metrics issue covers most of what those logs told you.