Add random 4-digit TV token per child for obfuscated TV URLs

Each child is assigned a unique permanent tv_token on creation. The TV
dashboard URL (/tv/:tvToken) and WebSocket (/ws/:tvToken) now use this
token instead of the internal DB ID. Existing children are backfilled
on startup. README updated to reflect the change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/DashboardView.vue

@@ -38,7 +38,7 @@
         <div class="card tv-card">
           <div class="card-title">TV Dashboard</div>
           <p class="tv-desc">Open this on the living room TV for the full-screen view.</p>
-          <a :href="`/tv/${activeChild.id}`" target="_blank" class="btn-primary">
+          <a :href="`/tv/${activeChild.tv_token}`" target="_blank" class="btn-primary">
             Open TV View →
           </a>
         </div>