Re-embed the knowledge base when the embedding model changes

Vectors from two different embedding models are not comparable — Titan v2 has
1024 dimensions where nomic-embed-text has 768 — and _cosine_similarity zips
them, so a mismatch produced quietly meaningless scores rather than an error.
Nothing forced a re-embed either: sync_local_kb only looked at the content
hash, so switching models left every cached vector in place.

local_knowledge_docs now records the model each vector came from, and a row is
re-embedded when its content hash OR its model differs. Saving a new
LOCAL_KB_EMBEDDING_MODEL from the settings panel starts a background pass over
every file; the knowledge base panel also has a button for it. While that pass
runs, the per-request sync steps aside so no file is embedded twice.

The panel now shows what the embedding model is doing: model name, vector
size, how many files are embedded with the current model, how many are still
on an older one, a progress bar while a pass runs, and the last error. A
"接続テスト" button embeds one short string so a broken or unreachable model
can be told apart from one that is merely slow to warm up.

Also read TAVILY_API_KEY from os.environ at call time. It was imported as a
config constant, which is evaluated before load_into_env() runs, so a key
saved from the settings panel was stored but never used — not even after a
restart.

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

docs/architecture.md

@@ -65,7 +65,7 @@ backend/
 | `memory_tags` | id, memory_id, tag, created_at | 要約への自動タグ |
 | `knowledge_entries` | id, query, summary, created_at | Web 検索で得た知識（セッション横断） |
 | `knowledge_sources` | id, entry_id, title, url | 上記知識の出典 URL |
-| `local_knowledge_docs` | id, path, title, content_hash, embedding_json, updated_at | ローカル Markdown KB の埋め込みキャッシュ |
+| `local_knowledge_docs` | id, path, title, content_hash, embedding_json, embedding_model, updated_at | ローカル Markdown KB の埋め込みキャッシュ（`embedding_model` が現在の設定と違う行は埋め直す） |
 | `note_pages` | id, title, content, position, section, created_at, updated_at | ユーザーと AI が共同編集する共有ノートのページ（`section` でタブをグループ化） |
 | `session_note_pages` | id, session_id, page_id, created_at | 「このセッションではこのページの本文をプロンプトに載せる」指定（session_id × page_id は一意） |
 | `app_settings` | key, value, updated_at | GUI 設定画面から編集した接続情報 |