fix: Phase 0b - PostgreSQL Decimal type handling

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

PostgreSQL NUMERIC/DECIMAL columns return decimal.Decimal objects,
not float. These cannot be mixed in arithmetic operations.

Fixed 3 locations:
- Line 62: float(weight_row['weight']) * 32.5
- Line 153: float(weight_row['weight']) for protein_per_kg
- Line 202: float(weight_row['avg_weight']) for adequacy calc

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

backend/calculations/nutrition_metrics.py

@@ -59,7 +59,7 @@ def calculate_energy_balance_7d(profile_id: str) -> Optional[float]:
 
         # Simple TDEE estimate: bodyweight (kg) × 30-35
         # TODO: Improve with activity level, age, gender
-        estimated_tdee = weight_row['weight'] * 32.5
+        estimated_tdee = float(weight_row['weight']) * 32.5
 
         balance = avg_intake - estimated_tdee
 
@@ -106,7 +106,7 @@ def calculate_protein_g_per_kg(profile_id: str) -> Optional[float]:
         if not weight_row:
             return None
 
-        weight = weight_row['weight']
+        weight = float(weight_row['weight'])
 
         # Get protein intake
         cur.execute("""
@@ -150,7 +150,7 @@ def calculate_protein_days_in_target(profile_id: str, target_low: float = 1.6, t
         if not weight_row:
             return None
 
-        weight = weight_row['weight']
+        weight = float(weight_row['weight'])
 
         # Get protein intake last 7 days
         cur.execute("""
@@ -199,7 +199,7 @@ def calculate_protein_adequacy_28d(profile_id: str) -> Optional[int]:
         if not weight_row or not weight_row['avg_weight']:
             return None
 
-        weight = weight_row['avg_weight']
+        weight = float(weight_row['avg_weight'])
 
         # Get protein intake (28d)
         cur.execute("""