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/templates/partials/log_form.html

@@ -1,9 +1,9 @@
 <!-- Inline logging form, included inside each exercise_card.html -->
 <form hx-post="/log"
-      hx-target="#logs-exercise-{{ exercise.id }}"
+      hx-target="#logs-exercise-{{ exercise_id }}"
       hx-swap="innerHTML"
       style="margin-bottom:0;">
-    <input type="hidden" name="exercise_id" value="{{ exercise.id }}">
+    <input type="hidden" name="exercise_id" value="{{ exercise_id }}">
     <input type="hidden" name="workout_day_id" value="{{ workout_day_id }}">
     <input type="hidden" name="set_number" value="{{ next_set|default(1) }}">