fix: Convert goal values to float before progress calculation

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

PostgreSQL NUMERIC columns return Decimal objects. Must convert
current_value, target_value, start_value to float before passing
to calculate_goal_progress_pct().

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

backend/calculations/body_metrics.py

@@ -400,6 +400,10 @@ def _score_weight_trend(profile_id: str) -> Optional[int]:
     if None in [current, target]:
         return None
 
+    # Convert Decimal to float (PostgreSQL NUMERIC returns Decimal)
+    current = float(current)
+    target = float(target)
+
     # If no start_value, use oldest weight in last 90 days
     if start is None:
         with get_db() as conn:
@@ -414,6 +418,8 @@ def _score_weight_trend(profile_id: str) -> Optional[int]:
             """, (profile_id,))
             row = cur.fetchone()
             start = float(row['weight']) if row else current
+    else:
+        start = float(start)
 
     # Progress percentage
     progress_pct = calculate_goal_progress_pct(current, target, start)