dashboard: live cache-age indicator

The web server's dashboardStats now reports how stale its shared cache
snapshot is — data.cache = { enabled, ttlMs, ageMs }, using the age of the
oldest dataset feeding the view (OLT-STATE, plus ONU-STATE under detailed
scan). cache.js gains ageOf(key).

The dashboard renders a live-ticking badge from it (renderCacheBadge):
  - within TTL  -> "◷ cached Ns ago" (info)
  - past TTL    -> same, warn-coloured (next load will refetch)
  - no cache    -> "● live" (Electron app, or PFW_CACHE_TTL_SECONDS=0)
The Dashboard Refresh button now sends fresh:true to bypass the cache and
reset the age; the detailed-stats toggle and auto-loads use the cache.

Shared renderer change, so the Electron app shows "live" (it sends no cache
field). Verified offscreen over HTTP: cached / stale / live states render
with the right text + colour.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/cache.js

@@ -51,6 +51,12 @@ class TtlCache {
     return p;
   }
 
+  /** Age (ms) of the cached entry for `key`, or null if not cached. */
+  ageOf(key) {
+    const e = this.entries.get(key);
+    return e ? Date.now() - e.at : null;
+  }
+
   /** Drop every entry whose key starts with `prefix` (e.g. one MCMS host's
    *  dataset). Returns how many were removed. */
   invalidate(prefix) {