Remove REST API, keep GraphQL as sole API

- Add missing GraphQL mutations: retryJob, updateAsset, deleteAsset
- Add UpdateAssetRequest and DeleteResult to schema source of truth
- Move Lambda callback endpoint to main.py (only REST endpoint)
- Remove REST routes, pydantic schemas, and deps
- Remove pydantic target from modelgen.json
- Update architecture diagrams and documentation

docs/architecture/01b-aws-architecture.dot

@@ -35,7 +35,7 @@ digraph aws_architecture {
         fillcolor="#f0f8e8"
 
         django [label="Django Admin\n/admin\nport 8701"]
-        fastapi [label="FastAPI + GraphQL\n/api + /graphql\nport 8702"]
+        fastapi [label="GraphQL API\n/graphql\nport 8702"]
         timeline [label="Timeline UI\n/\nport 5173"]
     }
 
@@ -65,10 +65,10 @@ digraph aws_architecture {
     browser -> nginx [label="HTTP"]
 
     nginx -> django [xlabel="/admin"]
-    nginx -> fastapi [xlabel="/api, /graphql"]
+    nginx -> fastapi [xlabel="/graphql"]
     nginx -> timeline [xlabel="/"]
 
-    timeline -> fastapi [label="REST API\nGraphQL"]
+    timeline -> fastapi [label="GraphQL"]
     django -> postgres
 
     fastapi -> postgres [label="read/write jobs"]