Multi-agent: add user_id column to jobs table

[chubes4]

Parent: #560

Summary

Add user_id BIGINT(20) UNSIGNED NOT NULL DEFAULT 0 column to datamachine_jobs table. Denormalized from flow/pipeline for direct query performance.

Code Changes

Schema

• inc/Core/Database/Jobs/ — add column to CREATE TABLE, add index
• Migration — ALTER TABLE + backfill from flow's user_id or admin user

Repository Methods

• create_job() — accept user_id param (set from execution context)
• get_jobs() / list methods — add optional user_id filter
• get_job() — no change (by ID)

CLI

• wp datamachine jobs list — add optional --user flag
• wp datamachine jobs show — no change (by ID)

Engine

• Job creation in engine — pull user_id from execution context and persist

Backward Compatibility

• DEFAULT 0, backfill to admin user
• Existing jobs are historical — backfill is informational