Implement RECEIPT_CLAIM transaction type

- Add status fields to Receipt model (status, claimed_at, claimed_by)
- Add RECEIPT_CLAIM handling to state_transition.py with validation and reward minting
- Add type field to Transaction model for reliable transaction type storage
- Update router to use TransactionRequest model to preserve type field
- Update poa.py to extract type from mempool transaction content and store only original payload
- Add RECEIPT_CLAIM to GasType enum with gas schedule

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

@@ -95,6 +95,7 @@ class Transaction(SQLModel, table=True):
     nonce: int = Field(default=0)
     value: int = Field(default=0)
     fee: int = Field(default=0)
+    type: str = Field(default="TRANSFER", index=True)
     status: str = Field(default="pending")
     timestamp: Optional[str] = Field(default=None)
     tx_metadata: Optional[str] = Field(default=None)
@@ -140,6 +141,9 @@ class Receipt(SQLModel, table=True):
     )
     minted_amount: Optional[int] = None
     recorded_at: datetime = Field(default_factory=datetime.utcnow, index=True)
+    status: str = Field(default="pending", index=True)  # pending, claimed, invalid
+    claimed_at: Optional[datetime] = None
+    claimed_by: Optional[str] = None
     
     # Relationship
     block: Optional["Block"] = Relationship(