feat: implement admin dashboard

Add Admin Dashboard at /admin for server configuration, disk usage
monitoring, and export job status, plus a public export/status endpoint.

Backend — new /api/v1/admin/* endpoints (RequireAdmin auth):
- GET  /admin/stats           → user/upload/comment counts + disk usage
- GET  /admin/config          → all config key/value pairs
- PATCH /admin/config         → update any subset of config keys; validates
                                 key whitelist and numeric values
- GET  /admin/export/jobs     → export_job rows for the event

Backend — public (AuthUser) endpoint:
- GET  /export/status         → released flag + zip/html job status/progress

Frontend — /admin page:
- Stats grid: guest count, upload count, comment count
- Disk usage bar with GB/MB formatting; red ≥ 90%, amber ≥ 75%
- Config form: labelled numeric inputs for all eight config keys,
  sends only changed values on save
- Export jobs list: type label, status badge, progress bar for running jobs,
  error message if failed; manual refresh button

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main.rs

@@ -69,7 +69,16 @@ async fn main() -> Result<()> {
         .route("/api/v1/host/users/{id}/unban", post(handlers::host::unban_user))
         .route("/api/v1/host/users/{id}/role", patch(handlers::host::set_role))
         .route("/api/v1/host/upload/{id}", delete(handlers::host::host_delete_upload))
-        .route("/api/v1/host/comment/{id}", delete(handlers::host::host_delete_comment));
+        .route("/api/v1/host/comment/{id}", delete(handlers::host::host_delete_comment))
+        // Export status (all authenticated users)
+        .route("/api/v1/export/status", get(handlers::admin::export_status))
+        // Admin Dashboard
+        .route("/api/v1/admin/stats", get(handlers::admin::get_stats))
+        .route(
+            "/api/v1/admin/config",
+            get(handlers::admin::get_config).patch(handlers::admin::patch_config),
+        )
+        .route("/api/v1/admin/export/jobs", get(handlers::admin::get_export_jobs));
 
     // Serve media files from disk
     let media_service = ServeDir::new(&config.media_path);