feat: add activity_detail placeholder for detailed activity logs

- New placeholder: {{activity_detail}} returns formatted activity log
- Shows last 20 activities with date, type, duration, kcal, HR
- Makes activity analysis prompts work properly

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

backend/placeholder_resolver.py

@@ -128,6 +128,35 @@ def calculate_age(dob: Optional[str]) -> str:
         return "unbekannt"
 
 
+def get_activity_detail(profile_id: str, days: int = 14) -> str:
+    """Get detailed activity log for analysis."""
+    with get_db() as conn:
+        cur = get_cursor(conn)
+        cutoff = (datetime.now() - timedelta(days=days)).strftime('%Y-%m-%d')
+        cur.execute(
+            """SELECT date, activity_type, duration_min, kcal_active, hr_avg
+               FROM activity_log
+               WHERE profile_id=%s AND date >= %s
+               ORDER BY date DESC
+               LIMIT 50""",
+            (profile_id, cutoff)
+        )
+        rows = [r2d(r) for r in cur.fetchall()]
+
+        if not rows:
+            return f"Keine Aktivitäten in den letzten {days} Tagen"
+
+        # Format as readable list
+        lines = []
+        for r in rows:
+            hr_str = f" HF={r['hr_avg']}" if r.get('hr_avg') else ""
+            lines.append(
+                f"{r['date']}: {r['activity_type']} ({r['duration_min']}min, {r.get('kcal_active', 0)}kcal{hr_str})"
+            )
+
+        return '\n'.join(lines[:20])  # Max 20 entries to avoid token bloat
+
+
 def get_trainingstyp_verteilung(profile_id: str, days: int = 14) -> str:
     """Get training type distribution."""
     with get_db() as conn:
@@ -174,6 +203,7 @@ PLACEHOLDER_MAP: Dict[str, Callable[[str], str]] = {
 
     # Training
     '{{activity_summary}}': get_activity_summary,
+    '{{activity_detail}}': get_activity_detail,
     '{{trainingstyp_verteilung}}': get_trainingstyp_verteilung,
 
     # Zeitraum