Persist layout download counts in PostgreSQL

## Summary
`POST /api/layouts/:id/use` increments downloads in an in-memory `Map` — counts reset on API restart.

## Tasks
- [ ] Add `layouts` table (id, download_count, metadata) or extend schema
- [ ] Drizzle migration / push
- [ ] Replace in-memory `incrementLayoutDownloads()`
- [ ] Seed built-in layout rows on startup or in `db:seed`

## Files
- `apps/api/src/services/layout-registry.ts`
- `apps/api/src/routes/layouts.ts`
- `apps/api/src/db/schema.ts`

Start by reading apps/api/src/db/schema.ts, then trace incrementLayoutDownloads() in apps/api/src/services/layout-registry.ts and its use from apps/api/src/routes/layouts.ts. Add the Drizzle migration or push step and seed built-in layout rows through startup or db:seed; done means download counts persist across API restarts and the use route updates PostgreSQL.

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