feat: Phase 0b - add avg_per_week_30d aggregation method

- Calculates average count per week over 30 days
- Use case: Training frequency per week (smoothed)
- Formula: (count in 30 days) / 4.285 weeks
- Documentation: .claude/docs/technical/AGGREGATION_METHODS.md

backend/goal_utils.py

@@ -407,6 +407,21 @@ def _fetch_by_aggregation_method(
             row = cur.fetchone()
             return float(row['max_value']) if row and row['max_value'] is not None else None
 
+        elif method == 'avg_per_week_30d':
+            # Average count per week over 30 days
+            # Use case: Training frequency per week (smoothed over 4.3 weeks)
+            days_ago = date.today() - timedelta(days=30)
+            params = [profile_id, days_ago] + filter_params
+            cur.execute(f"""
+                SELECT COUNT(*) as count_value FROM {table}
+                WHERE profile_id = %s AND {date_col} >= %s{filter_sql}
+            """, params)
+            row = cur.fetchone()
+            if row and row['count_value'] is not None:
+                # 30 days = 4.285 weeks (30/7)
+                return round(float(row['count_value']) / 4.285, 2)
+            return None
+
         else:
             print(f"[WARNING] Unknown aggregation method: {method}")
             return None