feat(graph): add GraphBackend abstract interface

backend/app/graph/__init__.py

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+# Populated in Task 4 once factory.py exists

backend/app/graph/base.py

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+"""Abstract graph backend interface."""
+from abc import ABC, abstractmethod
+from typing import Any, Dict, List, Optional
+
+
+class GraphBackend(ABC):
+    @abstractmethod
+    def create_graph(self, graph_id: str, name: str, description: str = "") -> None: ...
+
+    @abstractmethod
+    def set_ontology(self, graph_ids: List[str], entities: Dict[str, Any], edges: Dict[str, Any]) -> None: ...
+
+    @abstractmethod
+    def add_batch(self, graph_id: str, episodes: List[Any]) -> List[str]: ...
+
+    @abstractmethod
+    def get_episode(self, uuid_: str) -> Any: ...
+
+    @abstractmethod
+    def get_all_nodes(self, graph_id: str) -> List[Dict[str, Any]]: ...
+
+    @abstractmethod
+    def get_all_edges(self, graph_id: str) -> List[Dict[str, Any]]: ...
+
+    @abstractmethod
+    def get_node(self, uuid_: str) -> Dict[str, Any]: ...
+
+    @abstractmethod
+    def get_node_edges(self, node_uuid: str) -> List[Dict[str, Any]]: ...
+
+    @abstractmethod
+    def search(self, graph_id: str, query: str, limit: int = 10, scope: str = "edges") -> Dict[str, Any]: ...
+
+    @abstractmethod
+    def add_text(self, graph_id: str, data: str) -> None: ...
+
+    @abstractmethod
+    def delete_graph(self, graph_id: str) -> None: ...

backend/tests/test_graph_factory.py

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+import pytest
+from unittest.mock import MagicMock, patch
+
+
+def test_graph_backend_has_required_methods():
+    from backend.app.graph.base import GraphBackend
+    required = [
+        "create_graph", "set_ontology", "add_batch", "get_episode",
+        "get_all_nodes", "get_all_edges", "get_node", "get_node_edges",
+        "search", "add_text", "delete_graph",
+    ]
+    for method in required:
+        assert hasattr(GraphBackend, method), f"GraphBackend missing: {method}"

conftest.py

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+import sys
+import os
+
+# Allow `import backend.app.*` from the project root
+sys.path.insert(0, os.path.dirname(__file__))