REST API OpenAPI spec (#612)

* OpenAPI spec in specs/api.  Checked lint with redoc.

specs/api/components/schemas/rag/GraphRagResponse.yaml

@@ -0,0 +1,24 @@
+type: object
+description: Graph RAG response
+properties:
+  response:
+    type: string
+    description: Generated response based on retrieved knowledge graph
+    example: Quantum physics and computer science intersect in quantum computing...
+  end-of-stream:
+    type: boolean
+    description: Indicates streaming is complete (streaming mode)
+    default: false
+    example: true
+  error:
+    type: object
+    description: Error details if request failed
+    properties:
+      message:
+        type: string
+        description: Error message
+        example: Failed to retrieve graph data
+      type:
+        type: string
+        description: Error type
+        example: GRAPH_ERROR