webhook: use 200 instead of 201 for the default success response

[doc-han]

More conventional for a synchronous request that returns a body - 201 implies a created resource, which a webhook trigger doesn't map to.

Description

This PR [adds/changes/fixes]... (A description of your work goes here.)

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[github-actions]

Security Review ✅

• S0 (project scoping): N/A — the diff only changes a hard-coded HTTP status literal (run_channel.ex:483), no queries or project data access are touched.
• S1 (authorization): N/A — no new controller/channel/LiveView actions or policy changes; the existing gate on RunChannel is untouched.
• S2 (audit trail): N/A — no writes to project/instance configuration resources; only the default success status code is changed.