feat(graphrag): config.graph sub-model + settings + pgvector availability gate

GraphConfig (extraction_model?, max_chunks?, gleanings=0; extra=forbid) on SourceConfig; settings GRAPHRAG_EXTRACTION_MODEL (None=instance default) + GRAPHRAG_MAX_CHUNKS_FOR_EXTRACTION; graphrag_available() = GRAPHRAG_ENABLED and VECTOR_STORE==pgvector (D27-D32 foundation). Unit G1.

tests/graphrag/test_graphrag_available.py

@@ -0,0 +1,31 @@
+"""Tests for the graphrag_available() pgvector-gated flag (D29)."""
+
+from __future__ import annotations
+
+import pytest
+
+from application.graphrag import graphrag_available
+from application.core.settings import settings
+
+
+@pytest.mark.unit
+class TestGraphragAvailable:
+    def test_true_only_when_enabled_and_pgvector(self, monkeypatch):
+        monkeypatch.setattr(settings, "GRAPHRAG_ENABLED", True)
+        monkeypatch.setattr(settings, "VECTOR_STORE", "pgvector")
+        assert graphrag_available() is True
+
+    def test_false_when_disabled(self, monkeypatch):
+        monkeypatch.setattr(settings, "GRAPHRAG_ENABLED", False)
+        monkeypatch.setattr(settings, "VECTOR_STORE", "pgvector")
+        assert graphrag_available() is False
+
+    def test_false_when_store_not_pgvector(self, monkeypatch):
+        monkeypatch.setattr(settings, "GRAPHRAG_ENABLED", True)
+        monkeypatch.setattr(settings, "VECTOR_STORE", "faiss")
+        assert graphrag_available() is False
+
+    def test_false_when_disabled_and_not_pgvector(self, monkeypatch):
+        monkeypatch.setattr(settings, "GRAPHRAG_ENABLED", False)
+        monkeypatch.setattr(settings, "VECTOR_STORE", "faiss")
+        assert graphrag_available() is False