fix(seed): scale up data volume to make queries genuinely slow (>100ms)

The Performance page threshold is 100ms mean_exec_time. With the original
50K audit_log and 20K order_items rows, queries finished too fast to
exceed this threshold.

Changes:
- Add Step 4: Scale up data volume
  - audit_log: 50K → 300K+ rows
  - order_items: 20K → 100K+ rows
- Update Step 5 workload patterns to run enough iterations
- Add Step 12: Validation check that prints WARNING if slow-queries
  endpoint returns zero rows

The slow queries are now genuinely slow from real data scans:
- LOWER(status) on 5K orders
- Full table scan on 300K audit_log
- Large join across 100K order_items
- ILIKE with leading wildcard

NO pg_sleep or fabricated metrics.

Co-authored-by: Venkat SF <venkatesh.sakamuri@stayflexi.com>

Author: cursoragent