fix(poller): log watched contracts at startup

[sorinc03]

Summary

• Add one startup log line per configured contract using watching {label} on {network} ({contract_id})
• Include the contract's Horizon base URL as a structured log field
• Cover the startup message formatting and Horizon URL selection with focused tests

Testing

• git diff --check
• cargo fmt --check not run locally because cargo is not installed in this environment
• cargo test -p txwatch-poller not run locally because cargo is not installed in this environment and Docker daemon is not running

Fixes #124