Add section grouping to shared note tabs

Notes can now be clustered into named sections in the tab bar: each tab
gets a 🏷 button to assign it to a section, and clicking a section's
label renames (or, given an empty name, ungroups) every page in it.

note_pages gains a `section` column. Since the app has no migration
framework, database.py adds the column to existing SQLite files at
startup via PRAGMA table_info + ALTER TABLE. core/notes.set_section()
keeps same-section pages contiguous by renumbering position whenever a
page's section changes, so the tab bar and the AI-facing title list
(now grouped under "## section" headings) can both treat "adjacent" as
"same group" without a separate ordering field.

Also fixes a bug in the tab rename input: ref="renameInput" sits inside
a nested v-for, which Vue 3 binds as an array rather than a single
element, so renameInput.value?.select() was throwing on every rename.

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

backend/app/api/notes.py

@@ -4,7 +4,7 @@ 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.core.notes import next_position, rename_section, set_section
 from app.db.database import get_db
 from app.db.models import NotePage
 
@@ -16,6 +16,7 @@ class NotePageResponse(BaseModel):
     title: str
     content: str
     position: int
+    section: str | None
     updated_at: datetime
 
     class Config:
@@ -25,11 +26,18 @@ class NotePageResponse(BaseModel):
 class NotePageCreate(BaseModel):
     title: str = "新しいページ"
     content: str = ""
+    section: str | None = None
 
 
 class NotePageUpdate(BaseModel):
     title: str | None = None
     content: str | None = None
+    section: str | None = None
+
+
+class SectionRenameRequest(BaseModel):
+    old_name: str
+    new_name: str | None = None
 
 
 @router.get("", response_model=list[NotePageResponse])
@@ -44,9 +52,18 @@ def create_page(req: NotePageCreate, db: DBSession = Depends(get_db)):
     db.add(page)
     db.commit()
     db.refresh(page)
+    if req.section is not None and req.section.strip():
+        page = set_section(db, page.id, req.section)
     return page
 
 
+@router.post("/sections/rename")
+def rename_section_endpoint(req: SectionRenameRequest, db: DBSession = Depends(get_db)):
+    """old_name セクションに属する全ページを new_name セクションへ一括で付け替える。new_name が空ならグループ解除。"""
+    count = rename_section(db, req.old_name, req.new_name)
+    return {"renamed": count}
+
+
 @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)
@@ -61,6 +78,11 @@ def update_page(page_id: int, req: NotePageUpdate, db: DBSession = Depends(get_d
         page.content = req.content
     db.commit()
     db.refresh(page)
+
+    # section はページの並び順にも影響するため、変更があれば専用ロジックで反映する
+    if "section" in req.model_fields_set:
+        page = set_section(db, page_id, req.section)
+
     return page