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/exercise_card.html

@@ -38,6 +38,7 @@
             {% set next_set = logs|length + 1 %}
             {% include "partials/log_entry.html" %}
         {% else %}
+            {% set exercise_id = exercise.id %}
             {% set next_set = 1 %}
             {% include "partials/log_form.html" %}
         {% endif %}