fix: Add error logging to Phase 0b placeholder calculation wrappers

Problem: All _safe_* functions were silently catching exceptions and returning 'nicht verfügbar',
making it impossible to debug why calculations fail.

Solution: Add detailed error logging with traceback to all 4 wrapper functions:
- _safe_int(): Logs function name, exception type, message, full stack trace
- _safe_float(): Same logging
- _safe_str(): Same logging
- _safe_json(): Same logging

Now when placeholders return 'nicht verfügbar', the backend logs will show:
- Which placeholder function failed
- What exception occurred
- Full stack trace for debugging

Example log output:
[ERROR] _safe_int(goal_progress_score, uuid): ModuleNotFoundError: No module named 'calculations'
Traceback (most recent call last):
  ...

This will help identify if issue is:
- Missing calculations module import
- Missing data in database
- Wrong column names
- Calculation logic errors

backend/placeholder_resolver.py

@@ -477,6 +477,7 @@ def _safe_int(func_name: str, profile_id: str) -> str:
     Returns:
         String representation of integer value or 'nicht verfügbar'
     """
+    import traceback
     try:
         # Import calculations dynamically to avoid circular imports
         from calculations import scores, body_metrics, nutrition_metrics, activity_metrics, recovery_metrics, correlation_metrics
@@ -522,6 +523,8 @@ def _safe_int(func_name: str, profile_id: str) -> str:
         result = func(profile_id)
         return str(int(result)) if result is not None else 'nicht verfügbar'
     except Exception as e:
+        print(f"[ERROR] _safe_int({func_name}, {profile_id}): {type(e).__name__}: {e}")
+        traceback.print_exc()
         return 'nicht verfügbar'
 
 
@@ -537,6 +540,7 @@ def _safe_float(func_name: str, profile_id: str, decimals: int = 1) -> str:
     Returns:
         String representation of float value or 'nicht verfügbar'
     """
+    import traceback
     try:
         from calculations import body_metrics, nutrition_metrics, activity_metrics, recovery_metrics, scores
 
@@ -577,6 +581,8 @@ def _safe_float(func_name: str, profile_id: str, decimals: int = 1) -> str:
         result = func(profile_id)
         return f"{result:.{decimals}f}" if result is not None else 'nicht verfügbar'
     except Exception as e:
+        print(f"[ERROR] _safe_float({func_name}, {profile_id}): {type(e).__name__}: {e}")
+        traceback.print_exc()
         return 'nicht verfügbar'
 
 
@@ -584,6 +590,7 @@ def _safe_str(func_name: str, profile_id: str) -> str:
     """
     Safely call calculation function and return string value or fallback.
     """
+    import traceback
     try:
         from calculations import body_metrics, nutrition_metrics, activity_metrics, scores, correlation_metrics
 
@@ -609,6 +616,8 @@ def _safe_str(func_name: str, profile_id: str) -> str:
         result = func(profile_id)
         return str(result) if result is not None else 'nicht verfügbar'
     except Exception as e:
+        print(f"[ERROR] _safe_str({func_name}, {profile_id}): {type(e).__name__}: {e}")
+        traceback.print_exc()
         return 'nicht verfügbar'
 
 
@@ -616,6 +625,7 @@ def _safe_json(func_name: str, profile_id: str) -> str:
     """
     Safely call calculation function and return JSON string or fallback.
     """
+    import traceback
     try:
         import json
         from calculations import scores, correlation_metrics
@@ -647,6 +657,8 @@ def _safe_json(func_name: str, profile_id: str) -> str:
         else:
             return json.dumps(result, ensure_ascii=False)
     except Exception as e:
+        print(f"[ERROR] _safe_json({func_name}, {profile_id}): {type(e).__name__}: {e}")
+        traceback.print_exc()
         return '{}'