fix: convert PostgreSQL Decimal to float for math operations

Fixed TypeError when preparing AI prompt template variables.
PostgreSQL returns NUMERIC columns as decimal.Decimal, not float.

**Fixed in _prepare_template_vars:**
- Weight calculations (protein targets, delta)
- Nutrition averages (kcal, protein, fat, carbs)
- Activity totals (kcal_active)

All Decimal values now converted to float before math operations.

Error was: "TypeError: unsupported operand type(s) for *: 'decimal.Decimal' and 'float'"

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

backend/main.py

@@ -790,7 +790,7 @@ def _prepare_template_vars(data: dict) -> dict:
     # Weight trend summary
     if len(weight) >= 2:
         recent = weight[:30]
-        delta = recent[0]['weight'] - recent[-1]['weight']
+        delta = float(recent[0]['weight']) - float(recent[-1]['weight'])
         vars['weight_trend'] = f"{len(recent)} Einträge, Δ30d: {delta:+.1f}kg"
     else:
         vars['weight_trend'] = "zu wenig Daten"
@@ -815,15 +815,15 @@ def _prepare_template_vars(data: dict) -> dict:
     # Nutrition summary
     if nutrition:
         n = len(nutrition)
-        avg_kcal = sum(d.get('kcal',0) for d in nutrition) / n
+        avg_kcal = sum(float(d.get('kcal',0) or 0) for d in nutrition) / n
-        avg_prot = sum(d.get('protein_g',0) for d in nutrition) / n
+        avg_prot = sum(float(d.get('protein_g',0) or 0) for d in nutrition) / n
         vars['nutrition_summary'] = f"{n} Tage, Ø {avg_kcal:.0f}kcal, {avg_prot:.0f}g Protein"
         vars['nutrition_detail'] = vars['nutrition_summary']
         vars['nutrition_days'] = n
         vars['kcal_avg'] = round(avg_kcal)
         vars['protein_avg'] = round(avg_prot,1)
-        vars['fat_avg'] = round(sum(d.get('fat_g',0) for d in nutrition) / n,1)
+        vars['fat_avg'] = round(sum(float(d.get('fat_g',0) or 0) for d in nutrition) / n,1)
-        vars['carb_avg'] = round(sum(d.get('carbs_g',0) for d in nutrition) / n,1)
+        vars['carb_avg'] = round(sum(float(d.get('carbs_g',0) or 0) for d in nutrition) / n,1)
     else:
         vars['nutrition_summary'] = "keine Daten"
         vars['nutrition_detail'] = "keine Daten"
@@ -835,13 +835,14 @@ def _prepare_template_vars(data: dict) -> dict:
 
     # Protein targets
     w = weight[0]['weight'] if weight else prof.get('height',178) - 100
+    w = float(w)  # Convert Decimal to float for math operations
     vars['protein_ziel_low'] = round(w * 1.6)
     vars['protein_ziel_high'] = round(w * 2.2)
 
     # Activity summary
     if activity:
         n = len(activity)
-        total_kcal = sum(a.get('kcal_active',0) for a in activity)
+        total_kcal = sum(float(a.get('kcal_active',0) or 0) for a in activity)
         vars['activity_summary'] = f"{n} Trainings, {total_kcal:.0f}kcal gesamt"
         vars['activity_detail'] = vars['activity_summary']
         vars['activity_kcal_summary'] = f"Ø {total_kcal/n:.0f}kcal/Training"