Add settings-configurable cap on logs kept in memory (ticket 026)

Settings.max_logs (default None = unlimited) is pushed to the engine
via a new EngineMessage::SetMaxLogs, enforced on every ingest and
whenever the setting changes at runtime (no restart required).
Eviction drops the oldest (lowest-sequence) logs; sequence numbering
isn't renumbered and buckets already derived from an evicted log are
left in place. Exposed via PATCH /api/settings (max_logs: number |
null) and a NavMenu numeric field (blank = unlimited).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/main.rs

@@ -58,6 +58,10 @@ async fn main() {
 
     let (engine_tx, engine_rx) = mpsc::channel(1024);
     tokio::spawn(engine::run(engine_rx));
+    {
+        let max_logs = settings.lock().unwrap().max_logs;
+        let _ = engine_tx.send(engine::EngineMessage::SetMaxLogs(max_logs)).await;
+    }
 
     // Ticket 009: reload the active profile's param/metric declarations into the
     // engine BEFORE starting ingest below. Ordering matters: declarations must be