fix: reduce stats workload

[ohld]

Summary

• add targeted indexes for the high-cost user_meme_reaction stats paths
• make meme stats use indexable recent-reaction predicates and avoid recent scans for targeted meme recalculation
• reduce scheduled stats pressure and stop upload approvals from launching full source/stats recomputes
• cap default Gunicorn workers and skip Prefect event emission when app workers have no PREFECT_API_URL
• limit VK pagination to the requested post count

Validation

• ruff check touched files
• ruff format --check touched files
• py_compile touched files
• pytest tests/stats/test_meme.py tests/stats/test_engagement_score.py tests/stats/test_batch_stats.py -q
• pytest tests/tgbot/test_upload_moderation.py tests/tgbot/test_upload_observability.py -q