fix: correct confidence thresholds for 30-89 day range

Bug: 30 days with 29 data points returned 'insufficient' because
it fell into the 90+ day branch which requires >= 30 data points.

Fix: Changed condition from 'days_requested <= 28' to 'days_requested < 90'
so that 8-89 day ranges use the medium-term thresholds:
- high >= 18 data points
- medium >= 12
- low >= 8

This means 30 days with 29 entries now returns 'high' confidence.

Affects: nutrition_avg, and all other medium-term metrics.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/data_layer/utils.py

@@ -91,7 +91,8 @@ def calculate_confidence(
             else:
                 return "insufficient"
 
-        elif days_requested <= 28:
+        elif days_requested < 90:
+            # 8-89 days: Medium-term analysis
             if data_points >= 18:
                 return "high"
             elif data_points >= 12:
@@ -101,7 +102,7 @@ def calculate_confidence(
             else:
                 return "insufficient"
 
-        else:  # 90+ days
+        else:  # 90+ days: Long-term analysis
             if data_points >= 60:
                 return "high"
             elif data_points >= 40: