Add percentage-based backup outlier filtering for heterogeneous data

Root cause: IQR filtering fails with wide distributions (mixed room counts, property types).
Example: 9,459 NIS/sqm deal (68% below average) passed IQR with k=1.0 due to wide IQR.

Solution: Dual filtering approach - IQR + percentage backup

Changes:
- Add ANALYSIS_USE_PERCENTAGE_BACKUP config (default: true)
- Add ANALYSIS_PERCENTAGE_THRESHOLD config (default: 0.5 = 50%)
- Apply percentage backup after IQR when method=iqr
- Update outlier report with percentage backup parameters
- Update CLAUDE.md docs with dual filtering explanation

Behavior:
1. Hard bounds filter (1K-100K/sqm, 100K min total)
2. IQR filter (k=1.0) - catches mild outliers
3. Percentage filter (50% from median) - catches extreme outliers
4. Deal removed if flagged by ANY method

Example: 12K/sqm deal (60% below 30K median) now caught by percentage backup
even when IQR bounds are permissive due to heterogeneous data.

Tests: All 326 tests pass, manual verification confirms extreme outliers removed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

nadlan_mcp/govmap/outlier_detection.py

@@ -300,6 +300,36 @@ def filter_deals_for_analysis(
                 if is_outlier:
                     filters_to_remove[value_indices[i]] = True
 
+    # Step 4: Apply percentage-based backup filtering (catches extreme outliers in heterogeneous data)
+    # This runs in addition to IQR when enabled, providing a safety net for wide distributions
+    if config.analysis_use_percentage_backup and config.analysis_outlier_method == "iqr":
+        # Extract values for the specified metric (same logic as above)
+        if metric == "price_per_sqm":
+            values = [
+                deal.price_per_sqm
+                for i, deal in enumerate(deals)
+                if deal.price_per_sqm is not None and not filters_to_remove[i]
+            ]
+            value_indices = [
+                i
+                for i, deal in enumerate(deals)
+                if deal.price_per_sqm is not None and not filters_to_remove[i]
+            ]
+        elif metric == "deal_amount":
+            values = [deal.deal_amount for i, deal in enumerate(deals) if not filters_to_remove[i]]
+            value_indices = [i for i in range(len(deals)) if not filters_to_remove[i]]
+        else:
+            values = []
+            value_indices = []
+
+        if values:
+            percentage_outliers = detect_outliers_percent(
+                values, config.analysis_percentage_threshold
+            )
+            for i, is_outlier in enumerate(percentage_outliers):
+                if is_outlier:
+                    filters_to_remove[value_indices[i]] = True
+
     # Filter deals
     filtered_deals = [deal for i, deal in enumerate(deals) if not filters_to_remove[i]]
     outlier_indices = [i for i, should_remove in enumerate(filters_to_remove) if should_remove]
@@ -314,6 +344,12 @@ def filter_deals_for_analysis(
             "iqr_multiplier": effective_iqr_multiplier
             if config.analysis_outlier_method == "iqr"
             else None,
+            "percentage_backup_enabled": config.analysis_use_percentage_backup
+            if config.analysis_outlier_method == "iqr"
+            else None,
+            "percentage_threshold": config.analysis_percentage_threshold
+            if config.analysis_use_percentage_backup and config.analysis_outlier_method == "iqr"
+            else None,
             "metric": metric,
             "price_per_sqm_min": config.analysis_price_per_sqm_min,
             "price_per_sqm_max": config.analysis_price_per_sqm_max,