feat(onboarding): multipart file upload for blob artifacts

Add a real file-upload path so PLC projects (and other blob artifacts:
firmware images, source archives, mobile packages) can be delivered to
the agent instead of referencing a local path.

- agent: POST /api/v1/targets/{id}/artifacts/upload (axum multipart);
  stores bytes under {artifact_store_base_path}/uploads/{id}/... and
  $push-es the artifact (with stored_path + size_bytes) onto the target.
- dashboard: upload_target_artifact server fn (reqwest multipart) + wizard
  file input for blob artifact kinds, with a PLC format selector.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

compliance-agent/src/api/routes.rs

@@ -26,6 +26,10 @@ pub fn build_router() -> Router {
             "/api/v1/targets/{id}/artifacts",
             post(handlers::onboarding::add_artifact),
         )
+        .route(
+            "/api/v1/targets/{id}/artifacts/upload",
+            post(handlers::onboarding::upload_artifact),
+        )
         .route(
             "/api/v1/targets/{id}/applicable-scans",
             get(handlers::onboarding::applicable_scans_for_target),