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>

backend/app/core/config.py

@@ -9,6 +9,9 @@ MEMORY_SUMMARIZE_THRESHOLD: int = int(os.getenv("MEMORY_SUMMARIZE_THRESHOLD", "2
 CORS_ORIGINS: list[str] = os.getenv("CORS_ORIGINS", "http://localhost:5173").split(",")
 DEBUG: bool = os.getenv("DEBUG", "false").lower() in ("1", "true", "yes")
 
+# GUI設定画面（/api/settings）を保護する簡易パスワード。未設定の場合、設定画面は常に403で拒否する。
+SETTINGS_PASSWORD: str | None = os.getenv("SETTINGS_PASSWORD")
+
 # メッセージへの自動タグ付けに使うモデル（未設定ならDEFAULT_MODELを使用）
 TAGGING_MODEL: str | None = os.getenv("TAGGING_MODEL")