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/api/settings.py

@@ -0,0 +1,46 @@
+from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
+from sqlalchemy.orm import Session as DBSession
+
+from app.core import settings_store
+from app.core.auth import require_settings_password
+from app.db.database import get_db
+
+router = APIRouter(prefix="/api/settings", tags=["settings"], dependencies=[Depends(require_settings_password)])
+
+
+class SettingEntry(BaseModel):
+    key: str
+    is_set: bool
+    masked: str | None
+    overridden: bool
+
+
+class SaveSettingsRequest(BaseModel):
+    values: dict[str, str]
+
+
+@router.get("", response_model=list[SettingEntry])
+def list_settings(db: DBSession = Depends(get_db)):
+    return settings_store.list_masked(db)
+
+
+@router.put("", response_model=list[SettingEntry])
+def save_settings(req: SaveSettingsRequest, db: DBSession = Depends(get_db)):
+    try:
+        for key, value in req.values.items():
+            if not value:
+                continue  # 空欄は「変更なし」として無視する
+            settings_store.save(db, key, value)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e)) from e
+    return settings_store.list_masked(db)
+
+
+@router.delete("/{key}", response_model=list[SettingEntry])
+def clear_setting(key: str, db: DBSession = Depends(get_db)):
+    try:
+        settings_store.clear(db, key)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e)) from e
+    return settings_store.list_masked(db)