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/api/notes.py

@@ -0,0 +1,73 @@
+from datetime import datetime
+
+from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
+from sqlalchemy.orm import Session as DBSession
+
+from app.core.notes import next_position
+from app.db.database import get_db
+from app.db.models import NotePage
+
+router = APIRouter(prefix="/api/notes", tags=["notes"])
+
+
+class NotePageResponse(BaseModel):
+    id: int
+    title: str
+    content: str
+    position: int
+    updated_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class NotePageCreate(BaseModel):
+    title: str = "新しいページ"
+    content: str = ""
+
+
+class NotePageUpdate(BaseModel):
+    title: str | None = None
+    content: str | None = None
+
+
+@router.get("", response_model=list[NotePageResponse])
+def list_pages(db: DBSession = Depends(get_db)):
+    return db.query(NotePage).order_by(NotePage.position, NotePage.id).all()
+
+
+@router.post("", response_model=NotePageResponse, status_code=201)
+def create_page(req: NotePageCreate, db: DBSession = Depends(get_db)):
+    title = req.title.strip() or "新しいページ"
+    page = NotePage(title=title, content=req.content, position=next_position(db))
+    db.add(page)
+    db.commit()
+    db.refresh(page)
+    return page
+
+
+@router.patch("/{page_id}", response_model=NotePageResponse)
+def update_page(page_id: int, req: NotePageUpdate, db: DBSession = Depends(get_db)):
+    page = db.get(NotePage, page_id)
+    if not page:
+        raise HTTPException(status_code=404, detail="Note page not found")
+    if req.title is not None:
+        title = req.title.strip()
+        if not title:
+            raise HTTPException(status_code=400, detail="title is empty")
+        page.title = title
+    if req.content is not None:
+        page.content = req.content
+    db.commit()
+    db.refresh(page)
+    return page
+
+
+@router.delete("/{page_id}", status_code=204)
+def delete_page(page_id: int, db: DBSession = Depends(get_db)):
+    page = db.get(NotePage, page_id)
+    if not page:
+        raise HTTPException(status_code=404, detail="Note page not found")
+    db.delete(page)
+    db.commit()