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/log_service.py

@@ -149,6 +149,8 @@ class LogService:
     def delete_log(self, log_id: int) -> None:
         """Delete a log entry.
 
+        Removes the log and cleans up the parent session if no logs remain.
+
         Args:
             log_id: The log entry ID.
 
@@ -159,10 +161,22 @@ class LogService:
         if log is None:
             raise ValueError(f"WorkoutLog with id {log_id} not found")
 
+        session_id = log.session_id
         self._session.delete(log)
         self._session.commit()
         logger.info("log_deleted", log_id=log_id)
 
+        # Clean up orphaned session if no logs remain
+        remaining = self._session.exec(
+            select(WorkoutLog).where(WorkoutLog.session_id == session_id)
+        ).first()
+        if remaining is None:
+            ws = self._session.get(WorkoutSession, session_id)
+            if ws:
+                self._session.delete(ws)
+                self._session.commit()
+                logger.info("session_deleted_empty", session_id=session_id)
+
     def get_latest_logs_for_exercise(
         self,
         user_id: int,