Add session export/import as JSON

Sessions can now be downloaded as a JSON file containing the session
metadata and every message with its tags, and re-imported as a new
session from that same format.

GET /chat/sessions/{id}/export streams the session as a file download.
POST /chat/sessions/import validates the roles, then recreates the
session under a fresh UUID so importing never collides with an existing
session. The sidebar gets a per-row download button and an import button
that feeds a hidden file input.

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

backend/app/api/chat.py

@@ -2,12 +2,12 @@ import asyncio
 import uuid
 from datetime import datetime, timezone
 from fastapi import APIRouter, Depends, HTTPException
-from fastapi.responses import StreamingResponse
+from fastapi.responses import StreamingResponse, JSONResponse
 from pydantic import BaseModel
 from sqlalchemy.orm import Session as DBSession
 
 from app.db.database import get_db
-from app.db.models import Session, Message
+from app.db.models import Session, Message, MessageTag
 from app.core.memory_kb import memory as mem
 from app.core import llm
 from app.core.memory_kb import knowledge
@@ -47,6 +47,25 @@ class MessageResponse(BaseModel):
         from_attributes = True
 
 
+class ImportMessage(BaseModel):
+    role: str
+    content: str
+    created_at: datetime | None = None
+    tags: list[str] = []
+
+
+class ImportSessionMeta(BaseModel):
+    title: str | None = None
+    model: str | None = None
+    created_at: datetime | None = None
+    updated_at: datetime | None = None
+
+
+class ImportRequest(BaseModel):
+    session: ImportSessionMeta = ImportSessionMeta()
+    messages: list[ImportMessage]
+
+
 @router.get("/sessions", response_model=list[SessionResponse])
 def list_sessions(db: DBSession = Depends(get_db)):
     return db.query(Session).order_by(Session.updated_at.desc()).all()
@@ -69,6 +88,73 @@ def get_messages(session_id: str, db: DBSession = Depends(get_db)):
     ]
 
 
+@router.get("/sessions/{session_id}/export")
+def export_session(session_id: str, db: DBSession = Depends(get_db)):
+    """セッションのメタデータと全メッセージをJSONファイルとしてダウンロードする。"""
+    session = db.get(Session, session_id)
+    if not session:
+        raise HTTPException(status_code=404, detail="Session not found")
+
+    data = {
+        "session": {
+            "id": session.id,
+            "title": session.title,
+            "model": session.model,
+            "created_at": session.created_at.isoformat(),
+            "updated_at": session.updated_at.isoformat(),
+        },
+        "messages": [
+            {
+                "id": m.id,
+                "role": m.role,
+                "content": m.content,
+                "created_at": m.created_at.isoformat(),
+                "tags": [t.tag for t in m.tags],
+            }
+            for m in session.messages
+        ],
+    }
+    return JSONResponse(
+        content=data,
+        headers={"Content-Disposition": f'attachment; filename="session_{session.id}.json"'},
+    )
+
+
+@router.post("/sessions/import", response_model=SessionResponse, status_code=201)
+def import_session(req: ImportRequest, db: DBSession = Depends(get_db)):
+    """export で出力したJSON（またはその形式に沿ったJSON）から新規セッションを作成する。"""
+    if not req.messages:
+        raise HTTPException(status_code=400, detail="messages is empty")
+    for m in req.messages:
+        if m.role not in ("user", "assistant", "system"):
+            raise HTTPException(status_code=400, detail=f"invalid role: {m.role}")
+
+    title = req.session.title or req.messages[0].content[:40]
+    session = Session(
+        id=str(uuid.uuid4()),
+        title=title,
+        model=req.session.model or DEFAULT_MODEL,
+    )
+    if req.session.created_at:
+        session.created_at = req.session.created_at
+    session.updated_at = req.session.updated_at or datetime.now(timezone.utc)
+    db.add(session)
+    db.flush()
+
+    for m in req.messages:
+        msg = Message(session_id=session.id, role=m.role, content=m.content)
+        if m.created_at:
+            msg.created_at = m.created_at
+        db.add(msg)
+        db.flush()
+        for tag in m.tags:
+            db.add(MessageTag(message_id=msg.id, tag=tag))
+
+    db.commit()
+    db.refresh(session)
+    return session
+
+
 @router.delete("/sessions/{session_id}", status_code=204)
 def delete_session(session_id: str, db: DBSession = Depends(get_db)):
     session = db.get(Session, session_id)