feat(graph): add GraphBackendFactory singleton

backend/app/graph/__init__.py

@@ -1 +1,3 @@
-# Populated in Task 4 once factory.py exists
+from .factory import get_graph_backend
+
+__all__ = ["get_graph_backend"]

backend/app/graph/factory.py

@@ -0,0 +1,39 @@
+"""Graph backend factory — returns singleton based on GRAPH_BACKEND env var."""
+from typing import Optional
+
+from .base import GraphBackend
+from ..utils.logger import get_logger
+
+logger = get_logger('mirofish.graph.factory')
+
+_backend_instance: Optional[GraphBackend] = None
+
+
+def get_graph_backend() -> GraphBackend:
+    """Return the configured graph backend singleton."""
+    global _backend_instance
+    if _backend_instance is not None:
+        return _backend_instance
+
+    from ..config import Config
+    backend_type = Config.GRAPH_BACKEND
+    logger.info(f"Initializing graph backend: {backend_type}")
+
+    if backend_type == "zep":
+        from .zep_backend import ZepBackend
+        _backend_instance = ZepBackend()
+    elif backend_type == "graphiti":
+        from .graphiti_backend import GraphitiBackend
+        _backend_instance = GraphitiBackend()
+    else:
+        raise ValueError(
+            f"Unknown GRAPH_BACKEND='{backend_type}'. Valid values: 'zep', 'graphiti'."
+        )
+
+    return _backend_instance
+
+
+def reset_graph_backend() -> None:
+    """Reset singleton (useful for testing)."""
+    global _backend_instance
+    _backend_instance = None

backend/tests/conftest.py

@@ -0,0 +1,12 @@
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def reset_graph_factory_singleton():
+    """Reset the graph backend singleton before each test to avoid cross-test contamination."""
+    yield
+    try:
+        import backend.app.graph.factory as fmod
+        fmod._backend_instance = None
+    except ImportError:
+        pass

backend/tests/test_graph_factory.py

@@ -54,6 +54,38 @@ def test_zep_backend_raises_without_key():
         cfg_mod.Config.ZEP_API_KEY = orig
 
 
+def test_factory_returns_zep_by_default():
+    import backend.app.graph.factory as fmod
+    import backend.app.config as cfg
+    orig_backend = cfg.Config.GRAPH_BACKEND
+    orig_key = cfg.Config.ZEP_API_KEY
+    try:
+        cfg.Config.GRAPH_BACKEND = "zep"
+        cfg.Config.ZEP_API_KEY = "test-key"
+        fmod._backend_instance = None
+        backend_instance = fmod.get_graph_backend()
+        from backend.app.graph.zep_backend import ZepBackend
+        assert isinstance(backend_instance, ZepBackend)
+    finally:
+        cfg.Config.GRAPH_BACKEND = orig_backend
+        cfg.Config.ZEP_API_KEY = orig_key
+        fmod._backend_instance = None
+
+
+def test_factory_raises_on_unknown_backend():
+    import backend.app.graph.factory as fmod
+    import backend.app.config as cfg
+    orig = cfg.Config.GRAPH_BACKEND
+    try:
+        cfg.Config.GRAPH_BACKEND = "unknown"
+        fmod._backend_instance = None
+        with pytest.raises(ValueError, match="Unknown GRAPH_BACKEND"):
+            fmod.get_graph_backend()
+    finally:
+        cfg.Config.GRAPH_BACKEND = orig
+        fmod._backend_instance = None
+
+
 def test_config_graphiti_errors_when_missing():
     import backend.app.config as cfg_mod
     orig_backend = cfg_mod.Config.GRAPH_BACKEND