Add shared notes with tabbed pages editable by user and AI

Adds a note_pages table backing a tabbed notes panel where the user and
the model can both read and write. Only page titles (plus a short
preview) are injected into the system prompt on every turn; full page
content is kept out of context and fetched on demand via a read_note
tool, with write_note letting the model append to, replace, or create a
page.

Generalizes llm.resolve_web_search into resolve_tool_calls so the
tool-calling loop handles web_search and the new note tools through one
path. The chat stream emits a notes_updated SSE event when the model
edits a note so the frontend panel can refresh; local edits are tracked
as dirty and never clobbered by that refresh.

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

backend/app/core/config.py

@@ -37,6 +37,10 @@ LOCAL_KB_EMBEDDING_MODEL: str = os.getenv("LOCAL_KB_EMBEDDING_MODEL", "text-embe
 LOCAL_KB_TOP_K: int = int(os.getenv("LOCAL_KB_TOP_K", "3"))
 LOCAL_KB_MIN_SCORE: float = float(os.getenv("LOCAL_KB_MIN_SCORE", "0.75"))
 
+# 共有ノート（ユーザーとAIが共同編集するノート）
+# read_note ツールが1回に返す本文の最大文字数
+NOTES_TOOL_CONTENT_LIMIT: int = int(os.getenv("NOTES_TOOL_CONTENT_LIMIT", "4000"))
+
 # Amazon Bedrock
 # 以下はすべて LiteLLM(boto3) が os.environ から直接読み取るため、.env に書くだけで自動的に有効になる。
 # IAM経由（推奨）: AWS_ACCESS_KEY_ID / AWS_SECRET_ACCESS_KEY を設定する。