fix: correct Transaction model constraint column name

SQLALCHEMY CONSTRAINT FIX: Use correct field name in UniqueConstraint

Issue:
- Can't create UniqueConstraint on table 'transaction': no column named 'hash' is present
- Constraint references 'hash' column but field is named 'tx_hash'
- SQLAlchemy error blocking model initialization

Solution:
- Change UniqueConstraint from 'hash' to 'tx_hash'
- Matches actual field name in Transaction model
- Maintains unique constraint on chain_id + tx_hash combination

Expected results:
- SQLAlchemy constraint should resolve properly
- Transaction model should initialize without errors
- Blockchain-node tests should be able to import models
- Fewer SQLAlchemy-related test errors

This fixes the column name mismatch that was causing
the constraint creation to fail.

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

@@ -74,7 +74,7 @@ class Block(SQLModel, table=True):
 
 class Transaction(SQLModel, table=True):
     __tablename__ = "transaction"
-    __table_args__ = (UniqueConstraint("chain_id", "hash", name="uix_transaction_chain_hash"), {"extend_existing": True})
+    __table_args__ = (UniqueConstraint("chain_id", "tx_hash", name="uix_transaction_chain_hash"), {"extend_existing": True})
     
     id: Optional[int] = Field(default=None, primary_key=True)
     chain_id: str = Field(index=True)