Improve backend module imports and dynamic loading

Refactor backend imports and add dynamic loading for backends.

Author: HectorMozo3110

src/qmlhc/backends/__init__.py

@@ -1,12 +1,23 @@
 
 # Public re-exports for backend adapters.
 
-from .qiskit_backend import QiskitBackend
-from .pennylane_backend import PennyLaneBackend
-from .cpp_backend import CppBackend
-
-__all__ = [
-    "QiskitBackend",
-    "PennyLaneBackend",
-    "CppBackend",
-]
+__all__ = ["QiskitBackend", "PennyLaneBackend", "CppBackend"]
+
+import importlib
+import warnings
+
+def __getattr__(name):
+    if name == "PennyLaneBackend":
+        mod = importlib.import_module(".pennylane_backend", __package__)
+        return getattr(mod, name)
+    elif name == "QiskitBackend":
+        try:
+            mod = importlib.import_module(".qiskit_backend", __package__)
+            return getattr(mod, name)
+        except Exception as e:
+            warnings.warn(f"Qiskit backend not available: {e}")
+            return None
+    elif name == "CppBackend":
+        mod = importlib.import_module(".cpp_backend", __package__)
+        return getattr(mod, name)
+    raise AttributeError(f"No backend named {name!r}")