chore(documents): wire backfill script into deploy sequence

Adds db:backfill:doc-folders npm script. Run after the 0051 migration
applies. Idempotent; safe to re-run on interrupted deploys.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -20,6 +20,7 @@
     "db:reset": "tsx scripts/db-reset.ts --confirm",
     "db:reseed:realistic": "pnpm db:reset && pnpm db:seed:realistic",
     "db:reseed:synthetic": "pnpm db:reset && pnpm db:seed:synthetic",
+    "db:backfill:doc-folders": "tsx scripts/backfill-document-folders.ts",
     "test:e2e": "playwright test",
     "test:e2e:smoke": "playwright test --project=smoke",
     "test:e2e:exhaustive": "playwright test --project=exhaustive",