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

webapp/src/lib/useSettingsPanel.ts

@@ -123,6 +123,15 @@ export function useSettingsPanel() {
     [guarded],
   )
 
+  const setMaxLogs = useCallback(
+    (maxLogs: number | null) =>
+      guarded(async () => {
+        const next = await updateSettings({ maxLogs })
+        setSettings(next)
+      })(),
+    [guarded],
+  )
+
   const importCssEntry = useCallback(
     (filename: string, content: string) =>
       guarded(async () => {
@@ -174,6 +183,7 @@ export function useSettingsPanel() {
     profiles,
     error,
     setColorScheme,
+    setMaxLogs,
     importCssEntry,
     removeCssEntry,
     addProfile,