Pin notes to a chat, and import notes from files

Two additions to the shared notes, both about getting content in and out of
the prompt.

Per-chat pinning. Notes normally reach the model as a title list only, and
the body arrives just if the model decides to call read_note. A 📌 on the tab
now puts a page's full body in the system prompt for the chat that is open,
every turn, no tool round trip. The selection is per session (new table
session_note_pages, unique on session_id + page_id), so different
conversations carry different pages. The title list marks pinned pages so the
model does not read them twice, and bodies are capped per page by
SESSION_NOTES_CONTENT_LIMIT.

A new chat has no session row until its first message, so the front end
cannot PUT the selection yet. POST /chat/stream therefore accepts
note_page_ids and applies it before building the context, which makes the
pins count from the very first turn.

File import. Dropping .md/.txt on the editor - or picking them with the new
⬆ button - creates pages, and a .zip is unpacked into one page per text
entry with the first path component as the section. That is exactly the
shape the export writes, so a downloaded zip round-trips back in. Titles that
collide get a " (2)" suffix rather than overwriting, since restoring a backup
into a live workspace should not destroy anything. UploadFile needs
python-multipart, added to the lockfile.

Toasts grew a kind: the import success message was rendering in the red error
toast, as was the existing session-import message. Both are green now.

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

backend/app/core/config.py

@@ -41,6 +41,13 @@ LOCAL_KB_MIN_SCORE: float = float(os.getenv("LOCAL_KB_MIN_SCORE", "0.75"))
 # 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"))
 
+# アップロードで一度に取り込めるページ数と、1ファイル（ZIP内は1エントリ）あたりの最大バイト数
+NOTES_IMPORT_MAX_PAGES: int = int(os.getenv("NOTES_IMPORT_MAX_PAGES", "100"))
+NOTES_IMPORT_MAX_BYTES: int = int(os.getenv("NOTES_IMPORT_MAX_BYTES", "2000000"))
+
+# セッションに紐付けたページ1枚あたり、システムプロンプトに載せる本文の最大文字数
+SESSION_NOTES_CONTENT_LIMIT: int = int(os.getenv("SESSION_NOTES_CONTENT_LIMIT", "8000"))
+
 # read_knowledge ツールが1回に返す蓄積知識の本文の最大文字数
 KNOWLEDGE_TOOL_CONTENT_LIMIT: int = int(os.getenv("KNOWLEDGE_TOOL_CONTENT_LIMIT", "4000"))