Show each running job's average duration in the Settings Active processes list

[alfwro13]

Summary

• The Settings → System Updates & Power Active processes list now shows each running job's average duration beside its running time, e.g. (running 3 min · avg 12m 5s).
• scheduler_run_log.avg_duration_sec changes from an exponential moving average (alpha 0.3) to the plain mean of the job's last 20 successful runs in scheduler_run_history. The Workflow Monitor reads the same column, so both surfaces show one number. Failed runs are excluded; a job with no successful run keeps its previous value.
• db_helpers.refresh_job_average_duration() is the single calculation: called after every recorded run and once in migrate_db() so existing moving-average values are replaced at upgrade.
• GET /api/system/active-jobs gains active_job_avg_sec (same keys as active_jobs, null when no history). _mark_job_started() takes an optional job_id so the ETF predictor, Account Price Scraper and Account Auto Top-up jobs, whose names are built per account, resolve their own history; other jobs resolve through their canonical label.
• The 20-run window is now constants.SCHEDULER_RUN_HISTORY_LIMIT, used by the history read, the average and the weekly prune.

Test plan

• pytest tests/test_scheduler_engine.py tests/test_01_database_schema.py tests/test_02_api_get_endpoints.py tests/test_03_api_post_endpoints.py tests/test_accounts_api.py tests/test_accounts_engine.py tests/test_maintenance_engine.py tests/test_risk_orchestrator_engine.py (771 passed)
• Visual check on the Settings page while a job is running
• Note: the Workflow Monitor's overlap-risk conflict check now compares against this mean instead of the moving average, so its warnings may shift slightly.
• Note: CSS_VERSION is 5.157 here; Show a Restart pending banner in Settings when the pulled code is newer than the running service #243 sets 5.156, so whichever merges second needs a one-line conflict resolution on that line.