fix(logging): flatten structured fields in JSON log output

tracing-subscriber's .json() nests event fields under a "fields"
subobject by default, making the structured data awkward to query
downstream. Enable flatten_event so fields live at the top level.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: eliebleton-manomano

src/main.rs

@@ -373,6 +373,7 @@ async fn main() -> Result<()> {
             .with_env_filter(env_filter)
             .with_target(false)
             .json()
+            .flatten_event(true)
             .init(),
         LogFormat::Text => fmt().with_env_filter(env_filter).with_target(false).init(),
     }