fix: resolve template errors, orphaned sessions, and auth redirects

- Fix exercise_id undefined error in log_form.html by using scalar
  exercise_id instead of exercise.id object reference
- Clean up orphaned WorkoutSession records when all logs are deleted
- Filter empty sessions from dashboard stats (sessions, volume, streak)
- Replace broken HTTPException auth redirect with custom exception
  handler that properly returns 302 to /login

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

app/services/analytics_service.py

@@ -48,23 +48,28 @@ class AnalyticsService:
             Dict with keys: total_sessions, total_volume, total_sets,
             current_streak, last_workout_date.
         """
-        sessions = self._session.exec(
+        all_sessions = self._session.exec(
             select(WorkoutSession)
             .where(WorkoutSession.user_id == user_id)
             .order_by(WorkoutSession.date.desc())
         ).all()
-        total_sessions = len(sessions)
 
+        # Only count sessions that still have log entries
+        sessions = []
         total_volume = 0.0
         total_sets = 0
-        for ws in sessions:
+        for ws in all_sessions:
             logs = self._session.exec(
                 select(WorkoutLog).where(WorkoutLog.session_id == ws.id)
             ).all()
+            if not logs:
+                continue
+            sessions.append(ws)
             for log_entry in logs:
                 total_sets += 1
                 weight = _weight_to_float(log_entry.weight_used)
                 total_volume += log_entry.reps_completed * weight
+        total_sessions = len(sessions)
 
         current_streak = 0
         if sessions:
@@ -162,6 +167,9 @@ class AnalyticsService:
                 select(WorkoutLog).where(WorkoutLog.session_id == ws.id)
             ).all()
 
+            if not logs:
+                continue
+
             day_volume = sum(
                 log_entry.reps_completed * _weight_to_float(log_entry.weight_used)
                 for log_entry in logs