fix: preserve deployment cancellation details

[structName]

Summary

• avoid overwriting deployment stdout/stderr/progress when cancel and background cleanup update the same run concurrently
• update only cancellation status and completion time, then reload the latest run for the API response
• synchronize the cancellation regression test with executor startup so it tests the intended in-flight cancellation path

Root cause

The cancel handler saved a stale full-row deployment snapshot after cancelling the task context. If the background runner persisted context canceled first, the handler could overwrite stderr and other cleanup fields with empty values.

Validation

• cancellation regression test: 100 consecutive runs
• go test ./internal/api ./internal/database
• go test ./...
• npm --prefix web run build
• scripts/check-deployment-sensitive-data.sh
• git diff --check