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/utils/auth.py

@@ -17,6 +17,10 @@ from app.services.auth_service import AuthService
 
 logger = structlog.get_logger(__name__)
 
+
+class NotAuthenticatedError(Exception):
+    """Raised when a request lacks valid authentication."""
+
 # Cookie name for the admin session
 SESSION_COOKIE_NAME = "session"
 
@@ -77,11 +81,7 @@ def _login_redirect():
     """Create a redirect exception to the login page.
 
     Returns:
-        An HTTPException-compatible RedirectResponse.
+        A NotAuthenticatedError handled by a registered exception handler
+        in main.py that sends a 302 redirect to /login.
     """
-    from fastapi import HTTPException
-
-    response = RedirectResponse(url="/login", status_code=303)
-    exc = HTTPException(status_code=303, detail="Not authenticated")
-    exc.response = response
-    return exc
+    return NotAuthenticatedError()