fix: blockchain RPC mapper errors and missing contract stubs

- Fix SQLAlchemy relationship references in base_models.py (models.Transaction -> base_models.Transaction)
- Create contracts_stub.py for when contract_service module is unavailable
- Wire up contracts_stub as fallback in router.py imports
- Blockchain RPC now returns data instead of 503 errors

apps/blockchain-node/src/aitbc_chain/base_models.py

@@ -47,8 +47,8 @@ class Block(SQLModel, table=True):
         back_populates="block",
         sa_relationship_kwargs={
             "lazy": "selectin",
-            "primaryjoin": "and_(aitbc_chain.models.Transaction.block_height==Block.height, aitbc_chain.models.Transaction.chain_id==Block.chain_id)",
-            "foreign_keys": "[aitbc_chain.models.Transaction.block_height, aitbc_chain.models.Transaction.chain_id]"
+            "primaryjoin": "and_(aitbc_chain.base_models.Transaction.block_height==Block.height, aitbc_chain.base_models.Transaction.chain_id==Block.chain_id)",
+            "foreign_keys": "[aitbc_chain.base_models.Transaction.block_height, aitbc_chain.base_models.Transaction.chain_id]"
         }
     )
     receipts: List["Receipt"] = Relationship(
@@ -108,8 +108,8 @@ class Transaction(SQLModel, table=True):
     block: Optional["Block"] = Relationship(
         back_populates="transactions",
         sa_relationship_kwargs={
-            "primaryjoin": "and_(aitbc_chain.models.Transaction.block_height==Block.height, aitbc_chain.models.Transaction.chain_id==Block.chain_id)",
-            "foreign_keys": "[aitbc_chain.models.Transaction.block_height, aitbc_chain.models.Transaction.chain_id]"
+            "primaryjoin": "and_(aitbc_chain.base_models.Transaction.block_height==Block.height, aitbc_chain.base_models.Transaction.chain_id==Block.chain_id)",
+            "foreign_keys": "[aitbc_chain.base_models.Transaction.block_height, aitbc_chain.base_models.Transaction.chain_id]"
         }
     )
 

apps/blockchain-node/src/aitbc_chain/rpc/contracts_stub.py

@@ -0,0 +1,23 @@
+"""Stub contracts module for when contract_service is not available."""
+from typing import Any, Dict
+from fastapi import Request
+
+
+async def _stub(request: Request, *args, **kwargs) -> Dict[str, Any]:
+    return {"error": "Contract service not available", "status": "unavailable"}
+
+
+deploy_messaging_contract = _stub
+list_contracts = _stub
+deploy_contract = _stub
+call_contract = _stub
+verify_contract = _stub
+get_messaging_contract_state = _stub
+get_forum_topics = _stub
+create_forum_topic = _stub
+get_topic_messages = _stub
+post_message = _stub
+vote_message = _stub
+search_messages = _stub
+get_agent_reputation = _stub
+moderate_message = _stub

apps/blockchain-node/src/aitbc_chain/rpc/router.py

@@ -111,6 +111,22 @@ try:
     )
 except ImportError:
     _logger.warning("Contracts module not available")
+    from .contracts_stub import (  # type: ignore[assignment]
+        deploy_messaging_contract,
+        list_contracts,
+        deploy_contract,
+        call_contract,
+        verify_contract,
+        get_messaging_contract_state,
+        get_forum_topics,
+        create_forum_topic,
+        get_topic_messages,
+        post_message,
+        vote_message,
+        search_messages,
+        get_agent_reputation,
+        moderate_message,
+    )
 from .sync import (
     export_chain,
     import_chain,