feat(admin): add scan settings + system information to settings page
- Scanning section: auto-scan toggle (on/off) and poll interval input
with HTMX form that updates system_settings table
- System Information section: surfaces all hardcoded constants
(session duration, password policy, rate limits, worker pool,
sync queue, tombstone TTL, OPDS page size, CORS, etc.)
- New ScanSettingsData type, ScanSettingsSection partial template
- New HTMX endpoint: PUT /admin/settings/scan
- registerAdminSettingsRoutes for scan settings HTMX CRUD
- Fix bookhoord→bookhoard import typo in admin_library.go
john-okeefe
64ad10b3f2