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/db/models.py

@@ -104,6 +104,25 @@ class LocalKnowledgeDoc(Base):
     updated_at: Mapped[datetime] = mapped_column(DateTime, default=lambda: datetime.now(timezone.utc), onupdate=lambda: datetime.now(timezone.utc))
 
 
+class AppSetting(Base):
+    """GUIの設定画面から編集する接続情報（APIキー等）。app.core.settings_store が読み書きする。"""
+    __tablename__ = "app_settings"
+
+    key: Mapped[str] = mapped_column(String, primary_key=True)
+    value: Mapped[str] = mapped_column(Text)
+    updated_at: Mapped[datetime] = mapped_column(DateTime, default=lambda: datetime.now(timezone.utc), onupdate=lambda: datetime.now(timezone.utc))
+
+
+class ModelPreset(Base):
+    """モデル選択プルダウン（ModelSelector.vue）に表示するプリセット。GUIの設定画面から編集する。"""
+    __tablename__ = "model_presets"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    label: Mapped[str] = mapped_column(String)
+    value: Mapped[str] = mapped_column(String, unique=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=lambda: datetime.now(timezone.utc))
+
+
 class ApiLog(Base):
     __tablename__ = "api_logs"