Add GET endpoint to list all collaborative sessions

Allows frontend to retrieve history of all past sessions for easy access
to previous discussions and documents.

GET /api/collaborate returns:
- List of last 50 sessions (ordered by most recent first)
- Each session includes: id, prompt, format, status, created/completed dates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/src/routes/collaborate.js

@@ -45,6 +45,43 @@ router.post('/', async (req, res) => {
   }
 })
 
+/**
+ * GET /api/collaborate
+ * List all collaborative sessions
+ */
+router.get('/', (req, res) => {
+  try {
+    const db = require('../db/schema.js').default
+    const stmt = db.prepare(`
+      SELECT
+        id,
+        initial_prompt,
+        document_format,
+        status,
+        created_at,
+        completed_at
+      FROM collaborative_sessions
+      ORDER BY created_at DESC
+      LIMIT 50
+    `)
+    const sessions = stmt.all()
+
+    res.json({
+      sessions: sessions.map(s => ({
+        sessionId: s.id,
+        prompt: s.initial_prompt,
+        documentFormat: s.document_format,
+        status: s.status,
+        createdAt: s.created_at,
+        completedAt: s.completed_at
+      }))
+    })
+  } catch (error) {
+    console.error('Error listing sessions:', error)
+    res.status(500).json({ error: 'Failed to list sessions' })
+  }
+})
+
 /**
  * POST /api/collaborate/:id/start
  * Start the collaborative session