Add GUI settings panel for API keys, model presets, and embedding model

- Add a password-protected /api/settings endpoint (HTTP Basic auth,
  backend/app/core/auth.py) backed by a DB-stored allowlist of
  connection settings (OPENAI_API_KEY, ANTHROPIC_API_KEY, AWS creds,
  OLLAMA_API_BASE, TAVILY_API_KEY, LOCAL_KB_EMBEDDING_MODEL). Saved
  values override .env and apply immediately via os.environ, so
  Lightsail deployments no longer need SSH + `.env` edits to rotate
  keys. Values are always returned masked to the frontend.
- Replace ModelSelector.vue's hardcoded model list with a DB-backed
  /api/models CRUD API (read is public, writes reuse the settings
  password) so the chat model dropdown can be edited from the GUI;
  seed it once with the previous hardcoded presets on first boot.
- Fix local_knowledge.py reading LOCAL_KB_EMBEDDING_MODEL once at
  import time instead of per-call, which would have silently ignored
  GUI/DB overrides for the embedding model.
- Add SettingsPanel.vue (API keys + model management UI) and document
  the new SETTINGS_PASSWORD env var in .env.example and the Lightsail
  deploy guide.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/ModelSelector.vue

@@ -1,28 +1,29 @@
 <script setup lang="ts">
+import { computed, onMounted } from 'vue'
 import { useChatStore } from '../stores/chat'
+import { useSettingsStore } from '../stores/settings'
 
 const store = useChatStore()
+const settingsStore = useSettingsStore()
 
-const presets = [
-  { label: 'GPT-5.5', value: 'gpt-5.5' },
-  { label: 'GPT-5.4', value: 'gpt-5.4' },
-  { label: 'GPT-5.4 mini', value: 'gpt-5.4-mini' },
-  { label: 'Claude Sonnet 4.6', value: 'claude-sonnet-4-6' },
-  { label: 'Claude Haiku 4.5', value: 'claude-haiku-4-5-20251001' },
-  { label: 'Ollama llama3', value: 'ollama/llama3' },
-  { label: 'Ollama Gemma4', value: 'ollama/gemma4' },
-  { label: 'Gemma 4 12B (llama-server)', value: 'gpt-3.5-turbo' },
-  { label: 'Claude Sonnet 4.6 (Bedrock)', value: 'bedrock/jp.anthropic.claude-sonnet-4-6' },
-  { label: 'Llama 3 70B (Bedrock)', value: 'bedrock/meta.llama3-70b-instruct-v1:0' },
-  { label: 'Nova 2 Lite (Bedrock)', value: 'bedrock/jp.amazon.nova-2-lite-v1:0' },
-]
+// 選択中のモデルがプリセット一覧に無い場合（削除された/カスタム値）も表示が壊れないよう、
+// その場で追加のoptionとして補う。
+const options = computed(() => {
+  const presets = settingsStore.modelPresets
+  if (presets.some((p) => p.value === store.selectedModel)) return presets
+  return [...presets, { id: -1, label: store.selectedModel, value: store.selectedModel }]
+})
+
+onMounted(() => {
+  settingsStore.loadModelPresets()
+})
 </script>
 
 <template>
   <div class="model-selector">
     <label>モデル</label>
     <select v-model="store.selectedModel">
-      <option v-for="p in presets" :key="p.value" :value="p.value">{{ p.label }}</option>
+      <option v-for="p in options" :key="p.value" :value="p.value">{{ p.label }}</option>
     </select>
   </div>
 </template>