Fix nested AVG() functions in statistical queries

- Fixed correlation_analysis: Direct Pearson correlation formula
- Fixed regression_analysis: Direct linear regression calculations
- Fixed hypothesis_testing: Two-stage query to avoid nested AVG()
- Fixed outlier_detection: Two-stage query for Z-score method
- Fixed distribution_analysis: Two-stage query for std dev calculation

All 8 statistical functions now use simple, reliable SQL queries.
Testing shows: descriptive_statistics ✓, correlation_analysis ✓,
percentile_analysis ✓, outlier_detection ✓, regression_analysis ✓,
distribution_analysis ✓, moving_averages ✓

Only hypothesis_testing needs server restart to complete testing.

Author: neverinfamous