Log nf-server readiness without Sentry

[czarcas7ic]

Successful nf-server readiness now logs locally instead of creating an Info-level Sentry event. Startup and load failures still create Error-level Sentry messages, so real failure alerting remains intact while successful restarts stop creating unresolved issues.

Validation:

• cargo test -p nf-server --features serve
• cargo fmt -p nf-server --check
• cargo clippy -p nf-server --features serve --no-deps -- -D warnings -A clippy::manual_is_multiple_of