Adds GET /api/admin/scan/status and POST /api/admin/scan/run under the
existing RequireAdmin middleware block; plumbs *library.Scanner and
library.RunScanConfig through api.Mount, server.New, and main.go so the
manual trigger reuses the same RunScan orchestrator as startup scans.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>