Web sampler retirement

Arthexis no longer executes stored shell-style cURL samplers from the content app.

What changed

• The WebRequestSampler and WebRequestStep models were retired.
• The generic scheduler hook that executed apps.content.tasks.run_scheduled_web_samplers was removed.
• Historical WebSample and WebSampleAttachment rows are removed.

This keeps Arthexis aligned with the suite's role as an integration pivot: live collection should happen in a dedicated app or integration module with typed request settings and controlled HTTP clients, not by storing shell commands in the database.