Complete signed uploads in bulk

Completes up to 25 signed uploads in one request — the batch counterpart of POST /assets/{assetId}/complete, counted once against the workspace's rate limit instead of once per file. Every asset is verified and validated as the single completion does. Each entry echoes its index and asset_id and reports status ready, or failed with failure_message, exactly as the single completion would; ok is false with an error only when completion could not run for that asset — an id that is missing, deleted or outside this credential's write access reports not_found, the single completion's deliberate ambiguity. A failed entry never stops the rest, and a repeated id completes once and reports under each index. Static media reserved with project_id are surfaced on that project's canvas without stacking on one another; GLB Model3D assets are associated with the project but are not auto-placed. A malformed asset_id rejects the whole request. Mutating public API requests support an optional Idempotency-Key header for client retries; duplicate keys within two hours return idempotency_duplicate, and a batch whose every entry was rejected releases its key (see the Idempotency guide).