Expose ComfyUI run time on Enhance jobs as run_ms

Parse execution_start vs the end event from /history, store it when the
poller moves a job to ready/failed (never overwritten with null), and
return it as "run_ms" on every job endpoint.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/2026-10-30-200000_add_enhance_run_ms/down.sql

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+ALTER TABLE enhance_jobs DROP COLUMN run_ms;

migrations/2026-10-30-200000_add_enhance_run_ms/up.sql

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+ALTER TABLE enhance_jobs ADD COLUMN run_ms BIGINT;