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

docs/blockchain/README.md

@@ -1,7 +1,7 @@
 # Blockchain Documentation
 
 **Generated**: 2026-03-08 13:06:38
-**Last Updated**: 2026-04-20
+**Last Updated**: 2026-04-22
 **Total Files**: 3
 
 ## Documentation Files
@@ -9,6 +9,34 @@
 - [Adding gitea-runner as Third Blockchain Node (aitbc2)](adding_gitea_runner_as_third_node.md) - Complete guide for adding a third node to the AITBC blockchain network, including configuration steps, issues encountered, and verification procedures.
 - [Blockchain Synchronization Issues and Fixes](blockchain_synchronization_issues_and_fixes.md) - Documentation of synchronization issues between AITBC nodes and their resolutions.
 
+## Transaction Types
+
+The AITBC blockchain supports the following transaction types:
+
+- **TRANSFER**: Standard value transfer between accounts
+- **MESSAGE**: On-chain messaging (value=0, fee-only) - allows sending short text messages without balance transfers
+- **RECEIPT_CLAIM**: Claim rewards from job completion receipts
+- **GPU_MARKETPLACE**: GPU marketplace transactions (bids, offers, purchases)
+- **EXCHANGE**: Exchange transactions (orders, trades, swaps, liquidity)
+
+### MESSAGE Transaction Type
+
+The MESSAGE transaction type allows users to send short on-chain messages without affecting account balances. The message is stored in the transaction payload and only the fee is deducted from the sender's balance.
+
+**Usage:**
+```bash
+curl -X POST http://localhost:8006/rpc/transaction \
+  -H "Content-Type: application/json" \
+  -d '{"type":"MESSAGE","from":"address","to":"address","amount":0,"fee":1000,"nonce":1,"payload":{"message":"Hello blockchain!"},"sig":"signature"}'
+```
+
+**Characteristics:**
+- value must be 0
+- fee > 0
+- recipient can be any address (or special "null" address)
+- No balance transfers (only fee deduction)
+- Message stored in transaction payload
+
 ## Category Overview
 This section contains documentation related to blockchain node setup, synchronization, and network configuration.