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/settings.rs

@@ -68,6 +68,10 @@ pub struct Settings {
     pub color_scheme: ColorScheme,
     pub css_entries: Vec<CssEntry>,
     pub profiles: Profiles,
+    /// Maximum number of ingested logs kept in memory. `None` (the default)
+    /// means unlimited — matches today's unbounded retention.
+    #[serde(default)]
+    pub max_logs: Option<usize>,
 }
 
 impl Default for Settings {
@@ -80,6 +84,7 @@ impl Default for Settings {
                 active_profile: None,
                 profiles_dir: "profiles".to_string(),
             },
+            max_logs: None,
         }
     }
 }