Set the search summary model from the settings panel

Summarizing web search results falls back to DEFAULT_MODEL, so a local Ollama
model ends up doing it and the call can fail — which surfaces only as
"保存できる要約なし" in the chat. Make SEARCH_SUMMARY_MODEL settable at runtime:
add it to the settings allowlist and read it from os.environ on each call
(knowledge._summary_model), the way LOCAL_KB_EMBEDDING_MODEL already works.
Importing it as a config constant would keep the old value until a restart.

Show the current value of the settings that are not secrets. A model name
rendered as ••••qwen3 gives no way to tell what is in effect, so the store now
marks those keys (NON_SECRET_KEYS) and returns them unmasked; the API field is
renamed masked -> display_value and carries a secret flag that the panel uses
to pick the input type, replacing the list it kept on its own side.

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

backend/app/api/settings.py

@@ -12,7 +12,10 @@ router = APIRouter(prefix="/api/settings", tags=["settings"], dependencies=[Depe
 class SettingEntry(BaseModel):
     key: str
     is_set: bool
-    masked: str | None
+    # 秘匿キーはマスク済みの値、それ以外は現在値そのもの。未設定なら None
+    display_value: str | None
+    # 入力欄をパスワード扱いにするか（APIキー等は True）
+    secret: bool
     overridden: bool