feat: upgrade LangGraph to 1.x and migrate deprecated APIs

- Bump langgraph from 0.4 to 1.0+, langgraph-supervisor from 0.0.12 to 0.0.30+
- Bump langchain-core, langchain-anthropic, langchain-openai to 1.x
- Add langchain>=1.0 dependency for new create_agent location
- Migrate create_react_agent -> create_agent (prompt -> system_prompt)
- Fix create_supervisor positional arg to named agents= parameter
- Replace AsyncMock checkpointer with InMemorySaver in tests (v1 type validation)
- Update version references in README, ARCHITECTURE, eng-review-plan

backend/pyproject.toml

@@ -6,12 +6,13 @@ requires-python = ">=3.11"
 dependencies = [
     "fastapi>=0.115,<1.0",
     "uvicorn[standard]>=0.34,<1.0",
-    "langgraph>=0.4,<1.0",
-    "langgraph-supervisor>=0.0.12,<1.0",
+    "langgraph>=1.0,<2.0",
+    "langgraph-supervisor>=0.0.30,<1.0",
     "langgraph-checkpoint-postgres>=3.0,<4.0",
-    "langchain-core>=0.3,<1.0",
-    "langchain-anthropic>=0.3,<2.0",
-    "langchain-openai>=0.3,<1.0",
+    "langchain>=1.0,<2.0",
+    "langchain-core>=1.0,<2.0",
+    "langchain-anthropic>=1.0,<2.0",
+    "langchain-openai>=1.0,<2.0",
     "langchain-google-genai>=2.1,<3.0",
     "psycopg[binary,pool]>=3.2,<4.0",
     "pydantic>=2.10,<3.0",