fix(spammer): timestamp catch-up blocks when fetched

[westkite1201]

Summary

• Timestamp each catch-up block after it is fetched instead of reusing the first post-gap notification timestamp.
• Update the block event timestamp docs to cover both live notifications and catch-up scans.
• Add a WebSocket-backed regression test for catch_up_scan timestamping.

Fixes #128

Testing

• RED: cargo test -p spammer catch_up_scan_timestamps_each_fetched_block_individually -- --nocapture failed before the fix on stale timestamp reuse
• cargo fmt --all -- --check
• cargo test -p spammer
• cargo clippy -p spammer --all-targets --all-features -- -D warnings
• git diff --check