Add approximate heap/memory tracker for ingested logs (ticket 011)

Tracks a running total of ingested log bytes (approximated as each
log's raw UTF-8 text length), incremented on ingest and decremented
in evict_excess_logs when a log is dropped. Exposed via StatsDto's
new approx_bytes_total field on GET /api/stats, and displayed in
NavMenu's stats panel formatted with toLocaleString() (comma
separators).

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

webapp/src/components/NavMenu.tsx

@@ -87,6 +87,7 @@ export function NavMenu({ stats }: { stats: Stats | null }) {
               <div>
                 seq {stats.first_sequence ?? 0}–{stats.last_sequence ?? 0}
               </div>
+              <div>{stats.approx_bytes_total.toLocaleString()} bytes</div>
             </>
           ) : (
             <div>connecting…</div>