feat: add database migration script for projects

Add migration script to backfill existing sessions into projects:
- Scans session files from Claude Sessions directory
- Extracts unique project names from metadata.project field
- Creates projects with random icons and colors
- Links sessions to their respective projects in database
- Provides detailed progress reporting and summary

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

package.json

@@ -5,6 +5,7 @@
   "scripts": {
     "start": "node server.js",
     "dev": "node server.js",
+    "migrate:projects": "node scripts/migrate-to-projects.js",
     "test": "echo \"Error: no test specified\" && exit 1"
   },
   "keywords": [],