Introduce manual import workflow and download permission support. Adds a Prisma migration and schema field (users.download_access) to track per-user download access, and updates admin UI to toggle global and per-user download access. Implements new APIs: filesystem browse, manual-import endpoint, download-access settings, audiobook download-status, and on-demand download-token generation. Adds frontend components for manual import and related tests, plus documentation for the manual-import feature and the documentation-agent prompt. Key files: prisma/migrations/20260212000000_add_download_access_permission/migration.sql, prisma/schema.prisma, src/app/api/admin/filesystem/browse/route.ts, src/app/api/admin/manual-import/route.ts, src/app/api/admin/settings/download-access/route.ts, src/app/api/requests/[id]/download-token/route.ts, src/app/api/audiobooks/[asin]/download-status/route.ts, and updated admin users pages/components and permissions util.
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