Add weather caching, useless-tip redirect, clock widget, and IAM Bedrock auth

- Cache the latest fetched tomorrow weather and expose it via /api/system/weather/latest
- Add a header Clock widget showing date/time plus the cached weather summary
- Redirect useless-talk mentions from the weather bot to the useless-tips webhook
- Switch Bedrock auth from bearer token to standard IAM access key/secret

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

frontend/src/App.vue

@@ -7,6 +7,7 @@ import ModelSelector from './components/ModelSelector.vue'
 import LogViewer from './components/LogViewer.vue'
 import ToastList from './components/ToastList.vue'
 import MemoryMonitor from './components/MemoryMonitor.vue'
+import Clock from './components/Clock.vue'
 
 const showLogs = ref(false)
 </script>
@@ -21,6 +22,7 @@ const showLogs = ref(false)
       <header>
         <span class="title">PlayChat</span>
         <ModelSelector />
+        <Clock />
         <button class="log-toggle" :class="{ active: showLogs }" @click="showLogs = !showLogs">
           ログ
         </button>