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/api/client.ts

@@ -81,6 +81,72 @@ export async function deleteLog(logId: number): Promise<void> {
   await fetch(`${BASE}/logs/${logId}`, { method: 'DELETE' })
 }
 
+export interface SettingEntry {
+  key: string
+  is_set: boolean
+  masked: string | null
+  overridden: boolean
+}
+
+function authHeader(password: string): HeadersInit {
+  return { Authorization: `Basic ${btoa(`x:${password}`)}` }
+}
+
+export async function fetchSettings(password: string): Promise<SettingEntry[]> {
+  const res = await fetch(`${BASE}/api/settings`, { headers: authHeader(password) })
+  return parseJson<SettingEntry[]>(res, '設定取得')
+}
+
+export async function saveSettings(values: Record<string, string>, password: string): Promise<SettingEntry[]> {
+  const res = await fetch(`${BASE}/api/settings`, {
+    method: 'PUT',
+    headers: { ...authHeader(password), 'Content-Type': 'application/json' },
+    body: JSON.stringify({ values }),
+  })
+  return parseJson<SettingEntry[]>(res, '設定保存')
+}
+
+export async function clearSetting(key: string, password: string): Promise<SettingEntry[]> {
+  const res = await fetch(`${BASE}/api/settings/${key}`, {
+    method: 'DELETE',
+    headers: authHeader(password),
+  })
+  return parseJson<SettingEntry[]>(res, '設定クリア')
+}
+
+export interface ModelPreset {
+  id: number
+  label: string
+  value: string
+}
+
+export async function fetchModelPresets(): Promise<ModelPreset[]> {
+  const res = await fetch(`${BASE}/api/models`)
+  return parseJson<ModelPreset[]>(res, 'モデル一覧取得')
+}
+
+export async function createModelPreset(label: string, value: string, password: string): Promise<ModelPreset> {
+  const res = await fetch(`${BASE}/api/models`, {
+    method: 'POST',
+    headers: { ...authHeader(password), 'Content-Type': 'application/json' },
+    body: JSON.stringify({ label, value }),
+  })
+  return parseJson<ModelPreset>(res, 'モデル追加')
+}
+
+export async function updateModelPreset(id: number, label: string, value: string, password: string): Promise<ModelPreset> {
+  const res = await fetch(`${BASE}/api/models/${id}`, {
+    method: 'PUT',
+    headers: { ...authHeader(password), 'Content-Type': 'application/json' },
+    body: JSON.stringify({ label, value }),
+  })
+  return parseJson<ModelPreset>(res, 'モデル更新')
+}
+
+export async function deleteModelPreset(id: number, password: string): Promise<void> {
+  await fetch(`${BASE}/api/models/${id}`, { method: 'DELETE', headers: authHeader(password) })
+}
+
 export function streamChat(
   message: string,
   sessionId: string | null,