✨(entitlements) add Entitlements backend with Deploy Center support (#31)

This checks if the user has access to the app and can create calendars.

src/backend/core/entitlements/backends/base.py

@@ -0,0 +1,27 @@
+"""Abstract base class for entitlements backends."""
+
+from abc import ABC, abstractmethod
+
+
+class EntitlementsBackend(ABC):
+    """Abstract base class that defines the interface for entitlements backends."""
+
+    @abstractmethod
+    def get_user_entitlements(
+        self, user_sub, user_email, user_info=None, force_refresh=False
+    ):
+        """Fetch user entitlements.
+
+        Args:
+            user_sub: The user's OIDC subject identifier.
+            user_email: The user's email address.
+            user_info: The full OIDC user_info dict (backends may
+                extract claims from it).
+            force_refresh: If True, bypass any cache and fetch fresh data.
+
+        Returns:
+            dict: {"can_access": bool}
+
+        Raises:
+            EntitlementsUnavailableError: If the backend cannot be reached.
+        """