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/api.ts

@@ -90,6 +90,8 @@ export type ColorScheme =
 export interface Settings {
   color_scheme: ColorScheme
   active_profile: string | null
+  /** Maximum ingested logs kept in memory; `null` means unlimited. */
+  max_logs: number | null
 }
 
 export type CssEntryKind = 'imported' | 'linked'
@@ -263,8 +265,14 @@ export function fetchSettings(): Promise<Settings> {
   return getJson('/api/settings')
 }
 
-export function updateSettings(update: { colorScheme?: ColorScheme }): Promise<Settings> {
-  return sendJson('/api/settings', 'PATCH', { color_scheme: update.colorScheme })
+export function updateSettings(update: {
+  colorScheme?: ColorScheme
+  /** Omit to leave unchanged; `null` clears the cap back to unlimited. */
+  maxLogs?: number | null
+}): Promise<Settings> {
+  const body: Record<string, unknown> = { color_scheme: update.colorScheme }
+  if ('maxLogs' in update) body.max_logs = update.maxLogs
+  return sendJson('/api/settings', 'PATCH', body)
 }
 
 export function fetchCssEntries(): Promise<CssEntry[]> {