feat: brainstorm sessions, PDF document Q&A, embedding fixes, and UI improvements

- Add brainstorm feature with collaborative canvas, AI idea generation, live cursors, playback, and export
- Add PDF upload/extraction/ingestion pipeline with pgvector document search (RAG)
- Add document Q&A overlay with streaming chat and PDF preview
- Add note attachments UI with status polling, grid layout, and auto-scroll
- Add task extraction AI tool and agent executor improvements
- Fix NoteEmbedding missing updatedAt column, re-index 66 notes with 1536-dim embeddings
- Fix brainstorm 'Create Note' button: add success toast and redirect to created note
- Fix memory echo notification infinite polling
- Fix chat route to always include document_search tool
- Add brainstorm i18n keys across all 14 locales
- Add socket server for real-time brainstorm collaboration
- Add hierarchical notebook selector and organize notebook dialog improvements
- Add sidebar brainstorm section with session management
- Update prisma schema with brainstorm tables, attachments, and document chunks

memento-note/app/api/brainstorm/[sessionId]/activity/route.ts

@@ -0,0 +1,46 @@
+import { NextRequest, NextResponse } from 'next/server'
+import prisma from '@/lib/prisma'
+import { auth } from '@/auth'
+import { verifyParticipant } from '@/lib/brainstorm-collab'
+
+export async function GET(
+  request: NextRequest,
+  { params }: { params: Promise<{ sessionId: string }> }
+) {
+  const session = await auth()
+  if (!session?.user?.id) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+  }
+
+  try {
+    const { sessionId } = await params
+    const { isParticipant } = await verifyParticipant(sessionId, session.user.id)
+
+    if (!isParticipant) {
+      return NextResponse.json({ error: 'Not a participant' }, { status: 403 })
+    }
+
+    const activities = await prisma.brainstormActivity.findMany({
+      where: { sessionId },
+      orderBy: { createdAt: 'desc' },
+      take: 50,
+      include: {
+        user: { select: { id: true, name: true, image: true } },
+      },
+    })
+
+    return NextResponse.json({
+      success: true,
+      data: activities.map(a => ({
+        id: a.id,
+        action: a.action,
+        details: a.details ? JSON.parse(a.details) : null,
+        createdAt: a.createdAt,
+        user: a.user ? { name: a.user.name, image: a.user.image } : null,
+      })),
+    })
+  } catch (error) {
+    console.error('Error fetching activity:', error)
+    return NextResponse.json({ error: 'Failed to fetch activity' }, { status: 500 })
+  }
+}